Sonochemical Synthesis of Highly Fluorescent Ag Nanoclusters

ARTICLE Sonochemical Synthesis of Highly Fluorescent Ag Nanoclusters Hangxun Xu and Kenneth S. Suslick* School of Chemical Sciences, University of Illinois at UrbanaOChampaign, Urbana, Illinois 61801 ven smaller than nanoparticles, noble metal nanoclusters consist of several ABSTRACT Highly fluorescent, stable, water-soluble Ag nanoclusters have been successfully prepared via a E ϳ to 100 metal atoms and possess convenient sonochemical approach using a simple polyelectrolyte, polymethylacrylic acid (PMAA), as a capping sizes comparable to the Fermi wavelength agent. The optical and fluorescence properties of the Ag nanoclusters can be easily controlled by varying the of electrons (i.e., the de Broglie wavelength synthetic conditions, such as sonication time, stoichiometry of the carboxylate groups to Ag؉, and polymer of the electrons at the Fermi level: ϳ0.5 nm molecular weight. for Ag and Au). Such nanoclusters have received increasing attention in recent years because their optical, electrical, and chemi- KEYWORDS: fluorescence · nanoclusters · silver · sonochemistry · ultrasound cal properties are significantly different than their larger cousins (i.e., noble metal nanoparticles).1Ϫ5 These nanoclusters are clusters in aqueous solutions using a com- of significant interest because they provide mon polyelectrolyte, polymethylacrylic acid (PMAA), as the capping agent to stabilize the bridge between atomic and nanoparti- and protect the Ag nanoclusters in the cle behavior in noble metals. Silver and gold solution. nanoclusters are generally fluorescent upon Sonochemistry has found important syn- photoexcitation.2,6Ϫ9 Silver nanoclusters are thetic applications in materials especially fluorescent and have been chemistry.29,30 The chemical effects of high proved to be excellent fluorophores for intensity ultrasound derive primarily from chemical sensing, bioimaging, and single- acoustic cavitation: the formation, growth, molecule studies after the observation of and implosive collapse bubbles in a liquid fluorescence from Ag nanoclusters.10Ϫ17 irradiated with ultrasound.31,32 Localized hot The synthesis of Ag nanoclusters in spots with temperatures of ϳ5000 K and aqueous solutions, however, is difficult be- pressures of hundreds of bars can be gener- cause Ag nanoclusters tend to aggregate: ated during ultrasonic irradiation of wa- Ag nanoclusters will grow continuously, ter.32 As a consequence, highly reactive spe- forming larger nanoparticles and reducing ● ● ● cies, including HO2 ,H,OH, and perhaps their surface energy, until their growth is Ϫ eaq , are formed during aqueous stopped by preformed templates or cap- sonolysis,33Ϫ36 similar to the effects of ␥-ray ping agents. DNA, polymers, dendrimers, or deep-UV irradiation of aqueous solutions. polymer capsules, polymer microgels, and While there have been a number of excel- multiarm star polymers have each been lent studies on the preparation of noble used as protecting and stabilizing agents metal nanoparticles by sonochemical re- to prepare fluorescent Ag nanoclusters in ductions in aqueous or alcohol 18Ϫ27 aqueous solutions. Water-soluble solutions,37Ϫ42 to the best of our knowl- *Address correspondence to highly fluorescent Ag nanoclusters have edge, there is no prior report of the synthe- [email protected]. been prepared from the radiolytic, chemi- sis of ultrasmall metal nanoclusters using a Received for review May 6, 2010 cal, or photochemical reduction of silver sonochemical method. and accepted May 24, 2010. salts in aqueous solutions containing such Published online May 27, 2010. 2,28 capping agents. Here we present a new RESULTS AND DISCUSSION 10.1021/nn100987k and easily controlled sonochemical synthe- Sonochemical reduction of Agϩ still re- sis of water-soluble fluorescent Ag nano- quires the use of a template or capping © 2010 American Chemical Society www.acsnano.org VOL. 4 ▪ NO. 6 ▪ 3209–3214 ▪ 2010 3209 ARTICLE -Figure 1. (a) UV؊vis spectra and (b) fluorescence emission spectra of the solution containing PMAA and AgNO3 after in creasing length of sonication time; the excitation spectrum shown in (b) corresponds to the 90 min sample with an emission wavelength of 610 nm (inset: solution of the Ag nanoclusters illuminated by a UV lamp with 365 nm excitation). TEM images of as-prepared Ag nanoclusters from different lengths of sonication: (c) 60 min, (d) 90 min (inset shows a single magnified Ag nanocluster), and (e) 180 min. agent to prevent the aggregation of Ag nanoclusters also serves as OH● radical scavengers to prevent oxida- to form large Ag nanoparticles. PMAA (whose carboxy- tion of small silver clusters to form silver oxide.44,45 lic acid groups have a strong affinity for silver ions and Meanwhile, the secondary radicals formed from reac- silver surfaces) has been shown to be a versatile tem- tion of PMAA with OH● exhibit strong reducing power plate for preparing Ag nanoclusters under UV and produce Ag atoms associated with the polymer irradiation.25,43 The charged carboxylate groups pro- chains. vide stability for Ag nanoclusters and prevent further We demonstrate that sonication of aqueous AgNO3 growth of nanoclusters to large nanoparticles. PMAA solutions with dissolved PMAA results in the formation of Ag nanoclusters. To prepare Ag nanoclusters, a freshly prepared solution of AgNO3 was mixed with an aqueous PMAA (Mw ϭ 9500, sodium salt) solution. The molar ratio of carboxylate groups (from the methacrylic acid units) to Agϩ was 1:1. The pH value of the resulting solution was adjusted to 4.5 to form a compacted coil conformation of PMAA, which has been claimed to favor the formation of Ag nanoclusters.43 The solution was sparged with Ar for 2 h and then sonicated for various time intervals. As shown in Figure 1a, upon sonication, the initially colorless solution gradually turns pink (90 min) and then dark red (180 min). The resulting Ag nanoclusters are highly fluorescent (Figure 1b) and have been characterized by transmission electron mi- croscopy (TEM) (Figure 1cϪe), which shows that the Figure 2. Fluorescence emission spectra of sonochemically prepared Ag nanoclusters (90 min sonication) at different excitation sonochemically prepared Ag nanoclusters are less than wavelengths. 2 nm in diameter. These ultrasmall Ag nanoclusters are 3210 VOL. 4 ▪ NO. 6 ▪ XU AND SUSLICK www.acsnano.org ARTICLE Figure 3. (a) Absorption spectra and (b) fluorescence emission spectra of sonochemically prepared fluorescent Ag nano- -clusters (60 min sonication) from samples with different ratios of polymethylacrylic acid carboxylate groups to Ag؉,asla beled. The excitation spectra are identical to that of Figure 1b. more clearly visible with a dark-field image, as shown Furthermore, the Ag nanoclusters sonochemically pre- in Figure S1 in Supporting Information. pared are stable under Ar; essentially no change in fluo- The evolution of absorption and fluorescence of rescence intensity was observed even after 1 month the sonochemically prepared Ag nanoclusters with du- storage in the dark (under Ar). ration of sonication has proved interesting. Before son- There is a strong relationship between a Ag nano- ication, the solution showed no absorption in the range cluster size and its fluorescence excitation and emis- of 300Ϫ800 nm. After 10 min sonication, a peak around sion wavelengths.23,43 We clearly have a distribution of 440 nm appeared and further sonication increases the Ag nanocluster sizes present, as revealed in Figure 2, intensity of this peak. As sonication continues (Figure which shows the emission spectra of Ag nanocluster so- 1a), a shoulder at 520 nm grows in, the peak at 440 nm lutions at various excitation wavelengths. For excita- gradually disappears, the 520 nm peak gradually blue tion wavelengths between 450 and 570 nm, the maxi- shifts to 490 nm, and finally a new absorption peak mum emission is observed at an excitation wavelength around 390 nm begins to emerge after sonicating for of 510 nm. The emission maximum was found to shift 180 min. A similar trend has been observed in UV- to longer wavelengths with increasing excitation wave- irradiated formation of Ag nanoclusters in polymer mi- lengths, which confirms a distribution of Ag nanoclus- crogels.21 Absorbance in the region from 440 to 520 nm ters with different numbers of Ag atoms.23,43 Note that has been observed previously and ascribed to the for- essentially all Ag nanoclusters prepared to date by any mation of Ag nanoclusters, with shorter wavelength synthetic method show a similar phenomenon; that is, corresponding to smaller nanoclusters.21 The peak at the emission maximum shifts as the excitation wave- 390 nm is the characteristic surface plasmon band of length varies. The ability to control nanocluster distribu- larger Ag nanoparticles. tion to a single nanocluster size has not yet been A distinctive feature of our Ag nanoclusters is their achieved. Ϫ strong fluorescence, which is not observed for Ag nano- The stoichiometry of the carboxylate groups (RCO2 ) particles. An intense fluorescence emission band to Agϩ also plays an important role in the preparation around 610 nm is observed upon excitation at 510 nm of water-soluble fluorescent Ag nanoclusters. Figure 3 (Figure 1b). The maximum emission intensity occurs for shows the absorption and emission spectra of Ag nan- Ϫ ϩ Ag nanoclusters produced by sonication for 90 min. oclusters with different RCO2 /Ag ratios produced un- Prolonged sonication led to the gradual decrease of der the same sonication time with the same Agϩ con- Ϫ ϩ the fluorescence intensity as formation of large nonflu- centration. Excess carboxylate groups (RCO2 /Ag ϭ orescent Ag nanoparticles occurred (as confirmed in the 4/1 or 2/1) lead to the formation of smaller Ag nanoclus- absorption spectra plasmon band at 390 nm and by di- ters with an absorption peak around 440 nm, while Ϫ ϩ rect observation in the TEM, Figure 1e). The correspond- fewer carboxylate groups (RCO2 /Ag ϭ 1/4 or 1/8) ing fluorescence excitation spectrum at 90 min was re- yield larger Ag nanoclusters with an absorption peak Ϫ ϩ corded, which exhibited a peak at 510 nm and a weak around 520 nm.

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