Enhanced Emission from Photoactivated Silver Clusters Coupled with

J. Phys. Chem. C 2009, 113, 11751–11755

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Enhanced Emission from Photoactivated Silver Clusters Coupled with Localized Surface Plasmon Resonance† Baku Takimoto, Hideki Nabika, and Kei Murakoshi* DiVision of Chemistry, Graduate School of Science, Hokkaido UniVersity, Sapporo, 060-0810, Japan ReceiVed: February 27, 2009; ReVised Manuscript ReceiVed: March 23, 2009

We report the effect of local surface plasmon resonance (LSPR) of Ag nanoparticles on the emission properties of photoactivated Ag clusters. The prepared Ag nanoparticles showed strong surface enhanced Raman scattering (SERS) upon green laser excitation (514.5 nm), which indicates the appearance of an effective field enhancement. Along with the strong SERS, selective emission from the photoactivated Ag cluster was also observed only for the emissive Ag cluster that was resonant with the LSPR. The coupling with the LSPR also demonstrated the ability to observe the emission from only a single emissive site, even if several nonresonant emissive clusters are present in close proximity. Introduction Metal nanoparticles have attracted significant attention for several decades owing to their physical and chemical properties, which are different from those of bulk materials. In particular, for restricted-free electron metals such as Au, Ag, and Cu, the characteristic optical properties of localized surface plasmon resonance (LSPR) lead to very effective enhancement of the surface enhanced Raman scattering (SERS)1 and fluorescence (SEF).2 Exotic phenomenon, based on drastic changes in the optical response of these particulate systems, have prompted extensive research into the development of ultrasensitive analytical applications.3-5 All of the described optical properties are strongly dependent on the particle size. For example, by decreasing the particle size from a few tens of nanometers to a few nanometers, the SPR wavelength gradually shifts toward a shorter wavelength.6 With additional decrease in the particle size down to a cluster corresponding to a few atoms in size, there emerges the quantum size effect of electronic band splitting. This produces a bright emission, which could be used as a novel biomaker7 or for optical data storage8 in the future. The excitation and emission wavelengths of these clusters are very sensitive to their structures.9 In this context, it is important to control the cluster size and structure to produce a tailor-made emission with the desired wavelength and intensity. Recent bottom-up technologies, via chemical synthesis, have produced novel protocols to yield Ag clusters that exhibit an emission of the desired wavelength.10-13 The use of DNA as a protective agent prevented aggregation or growth of the Ag clusters, thus leading to successful control of the emission properties. Another approach to obtain the emissive Ag cluster is the top-down approach, in which a Ag cluster is formed from preformed Ag nanoparticles via photoactivation.8 By utilizing top-down technologies, such as electron-beam lithography, at the nanoparticle fabrication step, the position of the emissive site can be controlled to within a single nanometer resolution.14 This provides a great advantage toward novel applications that require highly precise control in the ordering of the active metal clusters having well-controlled emissive properties. Despite the fact that the ordering of the emissive cluster unit can be †

Part of the “Hiroshi Masuhara Festschrift”. * Corresponding author. E-mail: [email protected]

controlled by recently developed top-down technologies, it is still rather difficult to control the optical properties of the photoactivated Ag cluster on the preformed Ag nanoparticles. Indeed, previously reported experiments have revealed that the photoactivated Ag cluster on preformed Ag nanoparticles exhibited an emission spectrum that was a superposition of emissions with different wavelengths.8,15-18 The photoactivated Ag clusters on the preformed Ag nanoparticles have an additional intriguing feature of their optical properties. As mentioned above, Ag nanoparticles demonstrate a LSPR band in the visible region. Similarly, the photoactivated Ag cluster absorbs and emits photons in the visible region. This means that it is possible to control the emission properties through coupling between the energies of the LSPR and the Ag cluster electronic transition. In the present paper, we have investigated the effect of the LSPR of preformed Ag nanoparticles on the emission properties of the photoactivated Ag clusters. For this purpose, we have fabricated substrates with Ag nanoparticles of different sizes; that is, with different LSPR properties. The emission properties of the photoactivated Ag clusters were investigated and are discussed on the basis of the energy coupling between the LSPR and the Ag cluster. Experimental Section Ethanol, thioacetamide (TAA), and aminododecane (AD) were purchased from Wako Pure Chemical (Osaka, Japan) and used without further purification. Coverslips that were used as substrates were sonicated in acetone and Milli-Q water, each for 5 min, and then cleaned by immersing in 1:1 H2SO4/HNO3 for 5 min followed by boiling in Milli-Q water for 5 min. Cleaned coverslips were stored in water just before use. A periodic array of triangular Ag nanoparticles was prepared on the coverslip by the nanosphere lithography (NSL) technique.19 Polystyrene (PS) beads (Polyscience Inc., Warrington, PA) with diameters of 350 and 3000 nm were used for the preparation of the template. The Ag was vacuum-evaporated to a thickness of 30 nm. The obtained substrates were denoted NSL350 and NSL3000, according to the PS bead diameter used as the template. The extinction spectra of the prepared NSL substrates were recorded using a multichannel spectrometer (MCPD-2000, Ohtsuka Electronics, Osaka, Japan). The emission images were

10.1021/jp901818v CCC: $40.75  2009 American Chemical Society Published on Web 04/14/2009

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Figure 1. The chemical structure of (a) TA and (b) AD used in the adsorption experiment.

Figure 2. Extinction spectra of (blue) NSL350 and (red) NSL3000 substrates.

acquired from the objective-type (60×) total internal reflection fluorescence microscopy (TIRFM) (IX-71, Olympus, Tokyo, Japan) using a 532 nm excitation laser, in which the emission with the wavelength longer than 580 nm was collected. The emission spectra were obtained through a Raman microprobe spectrometer with excitation at 514.4 nm. The estimated probe size of the irradiation for the emission spectrum measurement was ∼1 µm. For the molecular adsorption experiment, 1 mM ethanol solutions of TAA or AD (Figure 1) were prepared, and a small amount of each solution was dropped onto the substrate. Results and Discussion The extinction spectra of the fabricated NSL substrates are shown in Figure 2. A strong band at 540 nm along with a weak band at 430 nm was observed for NSL350. The NSL substrates are known to exhibit these two characteristic peaks, similar to other triangle nanoparticles fabricated by other approaches.20-23 The strong and weak bands were attributed to in-plane dipole and quadrupole resonant absorptions, respectively. The excitation of the quadrupole resonance requires a higher energy than that for the dipolar resonance; thus, the quadrupole resonance absorption appears at a shorter wavelength. Another possible explanation for the shorter wavelength band is an out-of-plane dipole absorption, which is a characteristic band for disk-shaped nanoparticles.20,21 Contribution of the out-of-plane band to the observed spectrum could be minor, however, because of the perpendicular configuration against the optical pathway in the present observation. The weak band remained for NSL3000 (Figure 2), whereas the strong band disappeared. Previous work clearly demonstrated that increases in the size of the template PS beads for the NSL process resulted in a red-shift of the dipole resonance wavelength.19 In the case that the 3000 nm PS beads are used with the 30 nm thick deposition, the peak wavelength of the dipole absorption is expected to be longer than 1000 nm, which is outside the detection limit of the present system. Since the quadrupole resonance energy is not sensitive to the particle size compared to the dipole resonance,20 it was observed at the same wavelength as that for the NSL350. Another characteristic feature of NSL3000 is the appearance of a large increase in the extinction at wavelengths longer than 600 nm. Although our

Figure 3. (a, b) AFM and (c, d) emission images taken for (a, c) NSL350 and (b, d) NSL3000 substrates. Note that the lateral scale of (a) is different from the others.

experimental observation cannot separate individual contributions of the adsorption and the scattering component to the extinction spectra, a broad featureless spectrum of NSL3000 could be due to optical scattering induced by relatively large nanoparticles, whose size is close to the incident light wavelength. A flat feature at the wavelength region above 600 nm reflects the presence of nanoparticles with a variety of sizes and shapes. Successful fabrication of the NSL substrate was also confirmed by AFM images, in which a characteristic array of triangular nanoparticles was observed (Figure 3(a, b). The TIRFM images of both NSL substrates acquired in air are also shown in Figure 3(c, d). Both substrates exhibited strong emission in response to 532 nm laser excitation. The mechanism of the formation of an emissive site on a Ag nanoparticle upon laser irradiation has been explained by the photochemical creation of emissive Ag clusters from surface oxidized Ag, such as Ag2O and AgO.8 Although individual triangular Ag nanoparticles on NSL3000 are recognized as the emissive sites in the TIRFM image (Figure 3(d), those sites are not clearly shown on NSL350 (Figure 3(c). The distance between the individual triangular nanoparticles on NSL350 is below the optical diffraction limit; thus, the emission from individual nanoparticles on NSL350 was not spatially resolved in the TIRF image, giving a rather obscure image compared to that of NSL3000. Typical emission spectra acquired in air in response to 514.5 nm laser excitation are shown in Figure 4. On both NSL350 and NSL3000 substrates, several intense and sharp peaks were observed below 560 nm that are attributable to the SERS signals from a small amount of adsorbed carbonaceous contamination from the air during the fabrication process. The appearance of a SERS signal implies that a surface enhanced electromagnetic field was induced on both substrates by the 514.5 nm laser excitation. In addition to the SERS signal, broad emission bands from the Ag clusters were also observed. It should be noted that the two NSL substrates investigated in the present study exhibited the emission at different wavelengths despite being exposed to the same excitation wavelength. The emission was observed at 600 and 700-750 nm on NSL350 and NSL3000, respectively. The difference in the peak wavelength corresponds to the variation in the size of Ag clusters composed of a few

Effect of LSPR of Ag Nanoparticles

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Figure 4. Emission spectra acquired for (blue) NSL350 and (red) NSL3000. The spectra of each substrate was taken at a different emissive site.

Ag atoms and a few tens of Ag atoms.9 For example, it has been reported that the emission wavelength red-shifted with an increase in the number of Ag atom at the wavelength region above 400 nm; that is, ∼420-460 nm for Ag, 480 nm for Ag2, and ∼620-710 nm for Ag3 in Ar gas matrix. 24 The observed difference in the emission spectra between NSL350 and NSL3000 indicates that specific Ag clusters with different sizes and shapes respond optically upon light irradiation onto respective Ag nanoparticles. The emission selectivity can be defined by the optical matching between the LSPR and the Ag cluster. As we have seen in Figure 4, both NSL substrates show relatively strong SERS effects, indicating that the enhanced electromagnetic field is formed on individual Ag nanoparticles. The energy distributions of the electromagnetic field on respective substrates should correspond to each extinction observed at ∼500-600 nm and above 600 nm for NSL350 and NSL3000, respectively. The difference in the relative intensities of the Raman bands observed at NSL350 and NSL3000 reflects the optical properties of the Ag nanoparticles at the excitation wavelength. On these substrates, significant emission enhancement can be expected when the energies of the exciting and emitted electromagnetic fields are in resonance with the LSPR:25

I ∼ |E| 2[L(λex)]2[L(λem)]2β

(1)

where E is the incident electromagnetic field, L(λex) is the localfield factor at the excitation wavelength, L(λem) is the localfield factor at the emission wavelength, and β is a constant. According to this equation, resonances at both the excitation and emission wavelengths are important for the comprehensive enhancement, which has been experimentally proved.26 By comparing the extinction (Figure 2) and emission spectra (Figure 4), the emission was observed at the wavelength that matches

the LSPR wavelength; i.e., ∼500-600 nm and above 600 nm for NSL350 and NSL3000, respectively.This result suggests a contribution of a resonance between the Ag cluster and the LSPR at the emission wavelength, which would be superposed on the resonance enhancement at the excitation wavelength. The variation in the emission intensity noticed in both Figure 3 and Figure 4 should be brought from the fact that both L(λex) and L(λem) are sensitive to the LSPR condition. It is well-known that the change in the nanoparticle configuration can alter the local-field factor by several orders of magnitudes, which resulted in the variation in the emission intensity.27 On the other hand, a variation in the emission wavelength is caused by a variation in the structure of the clusters, such as size and anisotropy, as mentioned above. 9,13,28-30 Emission from the Ag clusters can be controlled by surface modification. Figure 5 depicts the time evolution in the emission characteristics of the Ag nanoparticles immersed in ethanol with and without surface modification reagents, such as TAA and AD. The TIRFM observation revealed the effect of the immersion (Figure 5(a, b)). No change or a slight increase in the emissivity was observed for NSL350 immersed in ethanol alone and in ethanol containing AD, whereas strong quenching was observed within 1 min in ethanol with TAA. A similar quenching effect in ethanol was found for NSL3000; however, this effect was much more apparent than that for NSL350 (Figure 5(b)). After immersion of NSL 3000 into pure ethanol for 3 min, the emission was effectively quenched. Quenching was also observed in ethanol containing AD after 9 min. These results indicate that the emission properties of the Ag clusters formed on the Ag nanoparticles can be controlled by surface adsorption, modification of molecules, or both. The intensity changes of a single emissive site were tracked on successive TIRFM images (Figure 5(c, d)). It is noteworthy that all of the emissive sites show an on/off blinking behavior, with the intensity of the emission being almost constant in the “on” state. This on/off blinking behavior suggests that the observed emission is from a single Ag cluster. One other possible explanation for the on/off blinking on TIRFM observation is the desorption/readsorption of the Ag nanoparticles during the observation. However, AFM observation clarified the periodic array of the NSL substrate was found to be unchanged after the experiments (data not shown). This eliminates the possibility of the contribution from the change in the adsorbed condition of the Ag nanoparticle on the substrate. In the presence of AD, the emission intensity and the blinking behavior of NSL350 were quite similar to those observed for NSL350 in pure ethanol. This similarity was also observed in the TIRFM images of both systems (Figure 5(a)). For NSL3000, effective quenching due to immersion was observed in the TIRFM image (Figure 5(b)) as a decrease in the frequency and duration of the blinking (Figure 5(d)). Observations of the intensity trajectories of a single site strongly suggest that the emission intensity from the Ag nanoparticles is determined by the number of the emissive Ag cluster sites and the duration of their “on” state. The quenching by the surface modification could be explained by the formation of a nonemissive complex between the Ag cluster and the adsorbed molecules. More strong quenching at NSL3000 than at NSL350 may reflect a higher reactivity of the emissive Ag cluster site on the Ag nanoparticles in NSL3000. In other words, the result shows that the Ag clusters with an emission peak around 700-800 nm more strongly interact with ethanol and AD than those with an emission around 550-600 nm. Characteristics in the size or the electronic states of the cluster may contribute to the difference in the interactions.

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Figure 5. TIRFM images of (a) NSL350 and (b) NSL3000 substrates. Typical intensity changes of a single emissive site acquired on the successive TIRFM images of (c) NSL350 and (d) NSL3000. The each trajectory was taken at a different emissive site.

Ethanol with TAA, however, does not show this difference, because the TAA molecules may be able to adsorb more strongly on the Ag clusters on both substrates. Observation of the blinking behavior suggests that just a single Ag cluster site on a Ag nanoparticle contributes to the emission, despite the expectation of multiple cluster formation on a Ag nanoparticle. Several emissive Ag clusters should be formed by photoirradiation. In the present system, we also observed stepwise intensity changes during ethanol immersion, which indicates the presence of a few emissive sites in close proximity (Figure 6). Nonetheless, multistep blinking was much less frequently observed as compared to single step blinking. Although single step blinking has been previously reported for chemically created Ag clusters,12 it has not been well-described for the system based on a photoactivated Ag cluster on preformed Ag nanoparticles. This is due to the difficulties in controlling the photoactivation rate. During photoactivation, both cluster formation and photobleaching proceed simultaneously. This creates difficulty in controlling the number of emissive Ag clusters on the preformed Ag nanoparticles. On the other hand, our substrates have been proven to enhance the cluster emission only when the optical energy matches with the LSPR. This situation leads to the most effectively enhanced emission, being the single emissive site, because the intensity of this kind of strongly enhanced emission overwhelms other weakly or nonenhanced emissive clusters, even if they are present in close proximity. Furthermore, a cascade effect would also be involved for the appearance of the single emissive site.31 When more than two LSPR sites with different internanoparticle distances coexist in close proximity, the excited energy is transferred into the “hottest spot” that exists on the surface of the nanoparticles. Although our present system

Figure 6. Intensity changes showing multistep intensity change observed on NSL350 in ethanol. Each trajectory was taken at a different emissive site.

is not constructed to induce an effective cascade effect, further experiments using well-defined Ag nanoparticles under controlled LSPR conditions will yield much information on the relationship between the LSPR and Ag cluster emission. The present observation of the blinking suggests that the adsorbed photon energy concentrates to just one single emissive site on Ag nanoparticles. The findings in the present paper offer a primitive and conceptual demonstration toward future versatility

Effect of LSPR of Ag Nanoparticles in controlling the intensity, wavelength, and position of photoactivated Ag clusters. Conclusion A selective emission coupled with LSPR was observed from the photoactivated Ag cluster on preformed triangular Ag nanoparticles. Comparison between the extinction and emission spectra revealed that the strong interaction at the emission wavelength between the Ag cluster and the LSPR was responsible for the observed wavelength-selective enhancement. Our experiments have proved that an emission with a desired wavelength can be extracted by tuning the LSPR energy of the Ag nanoparticles. Furthermore, the LSPR-assisted enhancement enables extraction of information from a single emissive site that is strongly resonant with the LSPR, even in the presence of many weakly or nonresonant clusters in close proximity. Our finding offers a novel protocol for controlling the intensity, wavelength, and position of the photoactivated Ag clusters by tuning the configuration of the preformed Ag nanoparticles. Acknowledgment. This work was supported in part by KAKENHI (Grant-in-Aid for Scientific Research) No. 19049003 on priority area “Strong Photon-Molecule Coupling Fields (470)” and No. 18750001 of Young Scientists (B) from MEXT, Japan. References and Notes (1) Qian, X.-M.; Nie, S. M. Chem. Soc. ReV. 2008, 37, 912–920. (2) Lakowicz, J. R.; Ray, K.; Chowdhury, M.; Szmacinski, H.; Fu, Y.; Zhang, J.; Nowaczyk, K. Analyst 2008, 133, 1308–1346. (3) Sawai, Y.; Takimoto, B.; Nabika, H.; Ajito, K.; Murakoshi, K. J. Am. Chem. Soc. 2007, 129, 1658–1662. (4) Sawai, Y.; Takimoto, B.; Nabika, H.; Murakoshi, K. Can. J. Anal. Sci. Spectrosc. 2007, 52, 142–149. (5) Sawai, Y.; Takimoto, B.; Nabika, H.; Ajito, K.; Murakoshi, K. Faraday Discuss. 2006, 132, 179–190. (6) Li, C.; Shuford, K. L.; Chen, M.; Lee, E. J.; Cho, S. O. ACS Nano 2008, 2, 1760. (7) Yu, J.; Patel, S. A.; Dickson, R. M. Angew. Chem., Int. Ed. 2007, 46, 2028–2030. (8) Peyser, L. A.; Vinson, A. E.; Bartko, A. P.; Dickson, R. M. Science 2001, 291, 103–106.

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