NANO LETTERS
Laser-Induced Growth and Deposition of Noble-Metal Nanoparticles for Surface-Enhanced Raman Scattering
2003 Vol. 3, No. 5 593-596
Erik J. Bjerneld,* Fredrik Svedberg, and Mikael Ka1 ll* Department of Applied Physics, Chalmers UniVersity of Technology, S-41296, Go¨ teborg, Sweden Received January 18, 2003; Revised Manuscript Received March 8, 2003
ABSTRACT We describe laser-induced growth and deposition of gold and silver nanoparticles for use as surface-enhanced Raman scattering (SERS) substrates. The nanoparticles form at a glass surface in contact with an aqueous solution with metal ions and a reducing agent if the interface is illuminated with visible laser light. The technique thus allows for positioning of metal nanoparticles in devices with limited accessibility for optical sensing by SERS.
The unique electronic, optical, and catalytic properties of noble-metal nanoparticles have attracted increasing interest in recent years. Metal nanoparticles of different sizes are usually synthesized by the aqueous reduction of metal ions at elevated temperatures.1,2 It is also possible to reduce the metal ions photochemically to make noble-metal nanoparticles of different sizes in aqueous,3,4 organic,5 and silica6 media or on photoactive titanium dioxide surfaces.7,8 Nanoparticle materials can subsequently be tailored by laserinduced particle aggregation9,10 and deposition.11 The lasercontrolled manipulation of particle size and shape,12 for example, for optical recording,13 has also been demonstrated. The power of lasers for patterning materials by ablation, deposition, or etching is well known.14 Here we show the versatility of laser-induced growth and deposition of noblemetal nanoparticles from aqueous solutions for use as SERS sensors. We have previously reported15 that visible laser irradiation (λ ) 514.5 or 647.1 nm) focused on a glass surface in contact with an aqueous solution containing Ag ions and a reducing agent results in the growth and deposition of Ag nanoparticles at the glass interface. Furthermore, it was shown that the Ag growth phenomenon was photoinduced (i.e., the growth rate is proportional to the laser power).15 The chief advantages of this laser-controlled particle synthesis will be demonstrated here. First, the photoeffect is of a general chemical nature. Ordinary cover glass (or mica) can be used, thus special photoactive surfaces such as titanium oxide7,8 are not required. We have not observed any limitation in the choice of reducing agents, and both Ag and Au particles * Corresponding authors. E-mail:
[email protected] and kall@ fy.chalmers.se. 10.1021/nl034034r CCC: $25.00 Published on Web 04/11/2003
© 2003 American Chemical Society
can be synthesized. Second, the synthesis can be monitored in situ by recording the surface-enhanced Raman (SERS) scattering of metal adsorbates and by inspection with the microscope. The growth is terminated either by turning off the laser or by removing the ionic solution. The detection of SERS spectra is aided by “self- positioning” of the particles in the focused laser spot due to the photochemical origin of the growth.15 Thus, the optical signal can be tuned in terms of Raman spectra and in terms of the size of the deposited metal spot. These combined factors allow for the optimization of the nanoparticle substrates for optical sensing. Third, the use of microscope objectives for illumination permits the fabrication of micrometer-sized metal structures in devices with limited accessibility. This is demonstrated by the photoproduction of Ag nanoparticles for SERS inside a glass capillary. Finally, laser-induced growth and deposition of noble-metal nanoparticles on a glass surface from ionic solutions is easy and requires no subsequent development or fixation steps. The experimental setup is shown in Figure 1. The following experimental conditions produced well-defined spots of Ag or Au nanofilms consisting of nanoparticle aggregates and intergrowths: AgNO3 or HAuCl4 was dissolved in purified water and mixed with citrate in a 1:1 ratio (0.5 mM). A droplet of the solution was placed inside a thin glass cell or a capillary. The sample was placed under the microscope coupled to a Raman spectrometer (Dilor XY800 or Renishaw 2000) and illuminated with a laser beam (λ ) 514.5 or 647.1 nm, P ) 1 µW-20 mW, beam-waist diameter Φ ≈ 1 µm) through a cover-glass-corrected microscope objective (40×, NA ) 0.55) focused at the glass-water interface. The particle growth and deposition was then
Figure 2. (Right) Image of a Ag-particle spot inside a glass capillary. The spot has an ellipsoidal shape due to spherical aberrations caused by the curved capillary interface. The Ag spot was grown in 15 s using a power of 3 mW (λ ) 514.5 nm). Scale bar ) 6 µm. (Left) SERS spectrum of R6G from the Ag spot; the concentration in solution was [R6G] ) 1.7 × 10-7 M.
Figure 1. Schematic illustration of the experimental setup. For laser-controlled synthesis and deposition of noble-metal nanoparticles, an aqueous solution with Ag ions and a reducing agent (0.5 mM) in contact with a glass capillary is irradiated with visible laser light. After the photoinduced deposition of Ag nanoparticle aggregates inside the glass capillary, the dye rhodamine 6G (R6G) was introduced and detected by a SERS measurement. (See Figure 2). The dark-field image of a deposited Ag spot on a flat glass substrate was obtained using white-light illumination.
monitored by inspection with the microscope and by simultaneous SERS measurements. The dark-field image in Figure 1 shows the light scattering from a typical Ag spot using white-light illumination. The heterogeneous aggregates and intergrowths in the Ag spot scatter light of different colors as a result of a variety of possible surface plasmon resonances. The height of a typical Ag spot was determined by AFM to be ∼50 nm.15 To demonstrate the applicability of the laser-induced nanoparticle synthesis and deposition, we made particles in a glass capillary, as shown in Figure 2. All synthesis experiments were performed in backscattering geometry in which the Ag-particle surface that is directly illuminated is facing the glass interface of the capillary. After synthesis and deposition, the SERS spectra showed that citrate, the reducing agent, was adsorbed on the Ag surface. We then removed the aqueous solution and injected a rhodamine 6G (R6G) solution into the capillary. The citrate spectrum, with a characteristic carboxylate vibration at 1390 cm-1,16 can 594
easily be distinguished from the well-known SERS spectrum of R6G.17 Although the sensitivity of the setup is far from optimized, we could easily detect R6G at a concentration of 10-8 M. In experiments when both citrate and R6G were present in the aqueous reducing-agent solution, both had equally intense SERS spectra for 514.5-nm excitation if the R6G concentration was 104 times more dilute than the citrate concentration. This is mainly due to an additional resonance amplification of R6G, which has a strong absorption band at ∼528 nm. The deposited metallic substrates for SERS are rough,15 and it is well known that the SERS intensity is a complicated function of the substrate roughness. However, it is possible to tune the morphology of the metal nanofilm for SERS measurements by terminating the nanoparticle growth at a time when the most optimal substrate for SERS has been synthesized. To illustrate this concept, typical SERS intensity versus time traces during particle growth are shown in Figure 3. Note that the reducing agent is present during the growth and that a broad SERS background is included in the integrated intensity. The initial growth rate tripled if the glass was coated with 3-aminopropyltrimethoxysilane (APTMS), which has a high affinity for metallic Ag.18 The most intense citrate SERS spectrum was observed after 20 s on the APTMS-coated surface. The intensity of the citrate SERS spectra subsequently decreased with time despite the continuous growth of the particle spot (to a final diameter of ∼5 µm and a height of ∼50 nm15). This shows that there is not a simple relationship between SERS intensity and particle spot size. We know from AFM images that the laser-induced particle synthesis leads to intergrowths of particles. It is likely that the metal spots become more flat in the late stages of growth and therefore exhibit less-intense SERS. As a measure of the quality of SERS spectra, we used the signal-tobackground ratio (SBR) of the analyte SERS signal. The best citrate SERS spectrum from particles on APTMS-coated glass had twice the SBR compared to the best spectrum in Nano Lett., Vol. 3, No. 5, 2003
Figure 3. SERS intensity growth on two surfaces: APTMS-coated glass (A) and ordinary cover glass (B). The reducing agent citrate was present during the growth. The most optimal conditions for SERS were obtained after different illumination times for the two surfaces (after 20 and 100 s, respectively); the corresponding citrate spectra are shown in the insets. The experimental conditions were λ ) 514.5 nm and 200 µW.
the time series from nontreated glass (see insets A and B in Figure 3), reflecting a more efficient SERS substrate morphology on the APTMS-coated glass. Also evident in Figure 3 is that citrate spectra differed in spectral details for the different glass substrates. The citrate spectra from nontreated glass have a broad feature at 1600 cm-1. We believe that this additional contribution comes from decomposition products of citrate in Ag-particle junctions, which are more likely to form on ordinary cover glass than on APTMS-coated glass because the APTMS coating immobilizes small and isolated clusters. In such junctions, we expect high local EM fields and enhanced photochemical formation of amorphous carbon (AC). As an interesting side note, inefficient ensemble averaging in many micro-Raman measurements leads to temporal and spectral SERS fluctuations, particularly for AC SERS,19 because of inhomogeneous broadening of Raman bands. This has also been observed on SERS substrates made by the photoactivation of Ag oxide.20 Here, however, no spectral or temporal SERS fluctuations were observed, only a slow increase or decrease in overall intensity. The substrates are thus suitable for ensemble SERS measurements using Raman spectrometers coupled to microscopes. We further investigated the aromatic molecule thiophenol. Self-assembled thiophenol layers were remarkably stable; we could use a power of 20 mW, corresponding to an irradiance of 1 MW/cm2, without observing any laser-induced effects on SERS spectra. Using thiophenol as the reducing agent (no citrate was present), we see in the inset in Figure 4, which shows the in-plane C-C stretch of the thiophenol ring, that no broadening occurred over time; only the background and the SERS intensity increase monotonically with time until the curve reaches its maximum value. The difference between the two time traces in Figure 4 is that half of the amount of thiophenol that was used (curve A) had been replaced by Nano Lett., Vol. 3, No. 5, 2003
Figure 4. Competition experiments with thiophenol and citrate. The intensity of thiophenol SERS from particles without citrate (A) increases monotonically with time before it reaches its maximum value. The Raman peaks exhibit no broadening with time, as shown in the inset (no offsets). In a mixture of citrate and thiophenol (B), citrate reduces Ag ions, but thiophenol binds to the Ag surface (λ ) 514.5 nm, 200 µW).
citrate (curve B). This doubled the growth rate and led to a subsequent decay in SERS intensity, but there was no trace of citrate in the spectra. This result indicates that the growth of the Ag spots was determined by the citrate concentration and that thiophenol subsequently adsorbs to the surface, replacing citrate, as shown by identical spectra A and B in Figure 4. Thus, the growth rate is, not unexpectedly, determined by the strongest reducing agent present whereas SERS spectra are governed by the molecule that binds the most strongly to the surface. The quality of the best SERS spectra in the time series in Figure 4, in terms of SBR, is more or less the same as for spectra obtained from large and dense colloidal aggregates of 100-nm Ag particles under the same experimental conditions. Thus, we conclude that the SERS sensors produced by laser-induced growth and deposition are just as sensitive as the most effective SERS substrate configuration that we have found from Ag particles made by aqueous reduction. The laser-induced synthesis and deposition of noble-metal nanoparticles for SERS described in this letter opens up many interesting possibilities for positioning of optical sensors in lab-on-a-chip and electronic devices. Acknowledgment. We gratefully acknowledge Dr. K. V. G. K. Murty for experimental assistance and expertise. We thank the Swedish Foundation for Strategic Research and the Swedish Research Council for financial support. References (1) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55-75 595
(2) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 1852. (3) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574-2579. (4) Pal, A.; Pal, T. J. Raman Spectrosc. 1999, 30, 199-204. (5) Sumida, T.; Murakoshi, K.; Yanagida, S. Chem. Lett. 1999, 7, 599600. (6) Tanahashi, I.; Mitsuyu, T. J. Non-Cryst. Solids 1995, 181, 77-82. (7) Poroshkov, V. P.; Gurin, V. S. Surface Sci. 1995, 331-333, 15201525. (8) Sudnik, L. M.; Norrod, K. L.; Rowlen, K. L. Appl. Spectrosc. 1995, 50, 422-424. (9) Satoh, N.; Hasegawa, H.; Tsujii, K.; Kimura, K. J. Phys. Chem. 1994, 98, 2143-2147. (10) Takeuchi, Y.; Ida, T.; Kimura, K. J. Phys. Chem. B 1997, 101, 13221327. (11) Niidome, Y.; Hori, A.; Takahashi, H.; Goto, Y.; Yamada, S. Nano Lett. 2001, 1, 365-369. (12) Link, S.; Burda, C.; Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 6152.
596
(13) Sugiyama, M.; Inasawa, S.; Koda, S.; Hirose, T.; Yonekawa, T.; Omatsu, T.; Takami, A. Appl. Phys. Lett. 2001, 79, 1528-1530. (14) Ba¨uerle, D. Laser Processing and Chemistry, 3rd ed.; Springer: Berlin, 2000. (15) Bjerneld, E. J.; Murty, K. V. G. K.; Prikulis, J.; Ka¨ll, M. ChemPhysChem 2002, 1, 116-119. (16) Siiman, O.; Bumm, L. A.; Callaghan, R.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983, 87, 1014-1023. (17) Hildebrandt, P.; Stockburger, M. J. Phys. Chem. 1984, 88, 59355944. (18) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science (Washington, D.C.) 1995, 267, 1629-1632. (19) Kudelski, A.; Pettinger, B. Chem. Phys. Lett. 2000, 321, 356362. (20) Buchel, D.; Mihalcea, C.; Fukaya, T.; Atoda, N.; Tominaga, J.; Kikukawa, T.; Fuji, H. Appl. Phys. Lett. 2001, 79, 620-622.
NL034034R
Nano Lett., Vol. 3, No. 5, 2003
