pubs.acs.org/Langmuir © 2009 American Chemical Society
Transparent Silver Microcrystals: Synthesis and Application for Nanoscale Analysis Tanja Deckert-Gaudig,*,† Florian Erver,‡ and Volker Deckert†,§ †
‡
ISAS- Institute for Analytical Sciences, Bunsen-Kirchhoff-Strasse 11, D-44139 Dortmund, Germany, Technische Universita.t Dortmund, Otto-Hahn-Strasse 6, D-44227 Dortmund, Germany, and §IPHT- Institute for Photonic Technology, Albert-Einstein-Strasse 9, 07745 Jena, Germany Received March 22, 2009. Revised Manuscript Received April 17, 2009
Micrometer-sized atomically flat silver nanoplates with a thickness of 10-20 nm were obtained in a one-pot reduction of silver nitrate with hydrazine sulfate without surfactants or polymers. The optically transparent triangles and hexagons have been proven to be excellent substrates for experiments with a tip-enhanced Raman transmission setup.
Silver and gold nanoparticles present an important group of metal surfaces because of their optical, physical, and chemical properties. In particular, the optical properties are governed by localized surface plasmons whose resonances are dependent on the size and shape of gold and silver nanoparticles. This phenomenon is the basis for many standard tools using silver and gold nanoparticles as a measurement surface.1,2 For instance, a wide range of rough noble nanoparticles display surface-enhanced Raman scattering (SERS) where the adsorbed molecules experience a signal enhancement up to 1014 (e.g., refs 3-6). Triangular silver nanostructures with a size smaller than 100 nm have been reported to be valuable in SERS spectroscopy7-9 as well as in metal-enhanced fluorescence.10 Simulations on 10-100 nm silver triangles have shown that these species have multiple resonances, which are associated with strong electromagnetic fields. The latter are located in the sharp corners extending over several nanometers. The resonances change, broaden, and are red-shifted with increasing particle size. Additionally, several resonances overlap, and any enhancement if present at all is rather uniformly distributed over the whole surface.11,12 These considerations led to the conclusion that micrometer sized silver and also gold nanoplates should provide constant electromagnetic field enhancement over a relatively large area *Corresponding author. Fax: +49-231-1392-120. Tel: +49-231-1392-252. E-mail:
[email protected]. (1) Sharma, J.; Imae, T. J. Nanosci. Nanotechnol. 2009, 9, 19–40. (2) Pastoriza-Santos, I.; Liz-Marzan, L. M. J. Mater. Chem. 2008, 18, 1724– 1737. (3) Qian, X.-M.; Nie, S. M. Chem. Soc. Rev. 2008, 37, 912–920. (4) Kneipp, J.; Kneipp, H.; Kneipp, K. Chem. Soc. Rev. 2008, 37, 1052–1060. (5) Barhoumi, A.; Zhang, D.; Tam, F.; Halas, N. J. J. Am. Chem. Soc. 2008, 130, 5523–5529. (6) Hering, K.; Cialla, D.; Ackermann, K.; Dorfer, T.; Moller, R.; Schneidewind, H.; Mattheis, R.; Fritzsche, W.; Rosch, P.; Popp, J. Anal. Bioanal. Chem. 2008, 390(1), 113–124. (7) Zhou, J.; An, J.; Tang, B.; Xu, S.; Cao, Y.; Zhao, B.; Xu, W.; Chang, J.; Lombardi, J. R. Langmuir 2008, 24, 10407–10413. (8) Tiwari, V. S.; Oleg, T.; Darbha, G. K.; Hardy, W.; Singh, J. P.; Ray, P. C. Chem. Phys. Lett. 2007, 446, 77–82. (9) Jia, H.; Xu, W.; An, J.; Li, D.; Zhao, B. Spectrochim. Acta A: Mol. Biomol. Spectrosc. 2006, 64, 956–960. (10) Aslan, K.; Lakowicz, J. R.; Geddes, C. D. J. Phys. Chem. B 2005, 109, 6247– 6251. (11) He, Y.; Shi, G. J. Phys. Chem. B 2005, 109, 17503–17511. (12) Kottmann, J. P.; Martin, O. J. F.; Smith, D. R.; Schultz, S. Opt. Exp. 2000, 6, 213–219.
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DOI: 10.1021/la901001n
without being close to nanometer scale corner effects. This is of particular interest if a uniform enhancement is required and/or mainly other material characteristics of the substrate are of interest. Furthermore, as most experimental setups use optical microscopes, nanoplates of such dimensions can be easily localized visually. If such metal nanoparticles possess smooth surfaces, molecules should be immobilized uniformly. Most smooth surfaces that provide a good immobilization basis, e.g., gold substrates, are opaque, which is a major obstacle for many optical detection methods. Ideally the syntheses of gold and silver nanoplates combine all the mentioned advantages for substrates. Recently, the synthesis of transparent ultra flat gold nanoplates was demonstrated and ascertained that those surfaces are the most suitable substrates for tip-enhanced Raman scattering (TERS) measurements of amino acids.13 TERS combines the advantages of surface-enhanced Raman spectroscopy and field enhancement due to plasmon resonances with the high lateral resolution capabilities of near-field optics. This technique demonstrated great potential in the characterization of samples with single-molecule detection limits.14-16 In order to match the material properties of the TERS tip, which is made mostly by silver evaporation or etching, the next step was to develop a procedure that provides micrometer-sized but still transparent (height ca. 20 nm) silver nanoplates. The height is of extreme significance since only transparent particles enable optical experiments in transmission mode to be used. The synthetic efforts toward these substrates should be kept minimal for making them readily available. Furthermore, no polymers or surfactants should be added to avoid residues on the metal surface. Such adherent contaminations can easily prevent highly sensitive analytical experiments. The size of the so far studied silver polygons was found between 20 and 500 nm with only one report on 1 μm nanoplates.17,18 Mostly known syntheses use spherical silver seeds as precursors, yielding the intended nanoparticles in a second step. A review provides a summary of the variety of different silver nanospecies.2 (13) (14) (15) (16) (17) (18)
Deckert-Gaudig, T.; Deckert, V. Small 2009, 4, 432–436. Bailo, E.; Deckert, V. Chem. Soc. Rev. 2008, 37, 921–930. Deckert-Gaudig, T.; Bailo, E.; Deckert, V. J. Biophotonics 2008, 5, 377–389. Bailo, E.; Deckert, V. Angew. Chem., Int. Ed. 2008, 47, 1658–1661. Chen, S.; Carroll, D. L. Nano Lett. 2002, 2, 1003–1007. Zou, X.; Ying, E.; Chen, H.; Dong, S. Colloids Surf., A 2007, 303, 226–234.
Published on Web 5/5/2009
Langmuir 2009, 25(11), 6032–6034
Letter Table 1. Results of the Reduction of AgNO3 with Hydrazine Sulfate in the Presence of Sodium Citrate with Different Amounts of Citric Acid reaction 1 2 3 4
citric acid/ μmol
plate size/ μm
plate height/ nm
defined particles per 10 10 μm
0 25 37.5 50
0.1-0.2 0.4-0.5 0.8-2 1-5
12-20 16 20-25 20-25
>300 >100 5-10 10-20
Figure 2. (a) AFM topography (baseline corrected) of a single silver nanoplate with corresponding cross section along the gray line as the inset. (b) Corresponding height profile on the silver plate.
Figure 1. SEM images of silver triangular and hexagonal plates isolated from reduction of AgNO3 with hydrazine sulfate and various amounts of citric acid: (a) 0 μL; (b) 50 μL.
Our approach was based on the one-pot reduction of silver nitrate with hydrazine19 and was optimized to our requirements for TERS experiments. In a typical procedure, 8.5 mg (0.05 mmol) of AgNO3 was dissolved in 5 mL of water. One milliliter of the solution was filled up to 100 mL with water, and 50 mg (0.17 mmol) of sodium citrate in 5 mL of water was inserted at once. Under vigorous stirring, 1.3 mg (0.01 mmol) of hydrazine sulfate in 5 mL of water acidified with x μL (x = 0, 25, 37.5, 50) of 1 M citric acid was added dropwise. The colorless solution was stirred at room temperature for 2-42 h. For characterization, 6 mL of the reaction mixture was centrifuged on cleaned glass slides for 10 min at 3000 rpm. (19) Ghader, S.; Mantegbian, M.; Kokabi, M.; Sarraf Mamoory, R. Pol. J. Chem. 2007, 81, 1555–1556.
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Ultrasonication for 10 min in water removed almost all spherical byproducts. After drying in a vacuum, the plates were used without further treatment and scanning electron microscopy (SEM) as well as atomic force microscopy (AFM) measurements were accomplished. The measurements showed that the number of particles, as expected, decreases drastically with increasing size of the nanoplates. For an overview, the results of the reactions are given in Table 1. In Figure 1a,b, SEM images of isolated silver nanoplates are shown. One can see clearly the increasing size of the nanoplates with increasing amount of added citric acid. Addition of citric acid slows down the reaction drastically to 42 h compared to 5 min using neutral hydrazine.19 This observation can be explained by the lower pH value, which decreases the redox potential of hydrazine. A smaller reducing power implies a slower reduction rate, consequently enabling the formation of larger silver particles. Furthermore, the edges of the particles are better defined when 50 μL citric acid were used. Further acidification of the reaction mixture did not influence the appearance of the nanoplates. In addition to citric acid and hydrazine, sodium citrate plays a decisive role in the reaction. The citrate anions are postulated to control the growth of the silver particles by acting as capping ligand.9,18,19 As it turned out, changing the concentration of the citrate improves neither the appearance nor the size of the silver polygons. Interestingly, the height of the plates does not increase in the same manner as does the size. These observations are in DOI: 10.1021/la901001n
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accordance with earlier experiments and can be explained with the two-dimensional growth process of the crystals.18 Information on the morphology of the nanoplates was obtained from AFM measurements. In Figure 2a, the topography image of a 4 μm silver nanoplate is shown. The inset gives the height profile along the gray line across the plate. From the zoom in Figure 2b, the roughness on the nanoplate can be deduced. The height of a typical nanoplate was determined to be below 20 nm with a roughness across the silver plate of about 500 pm (rootmean-square (rms)). According to the AFM measurements, the silver surface was not as homogeneous as compared to the gold nanoplates (rms 100-200 pm).13 This is most likely due to the lower oxidation stability of Ag in such dimensions. Because of this susceptibility, the substrates must be used at once after isolation or stored in an inert gas atmosphere. To avoid oxidation of the silver surface, an atomic layer deposition of a thin (
