J. Phys. Chem. B 1998, 102, 493-497
493
Screening and Enrichment of Metal Nanoparticles with Novel Optical Properties Steven R. Emory and Shuming Nie* Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405 ReceiVed: October 21, 1997; In Final Form: December 2, 1997
A rapid screening and enrichment method is reported for exploring the size and shape diversities (libraries) of nanometer-scale colloidal particles. With the use of surface-enhanced Raman scattering, a new class of metal nanoparticles has been identified from heterogeneous silver colloids. These particles are relatively large, faceted nanocrystals that are able to enhance the efficiencies of surface optical processes by as much as 14-15 orders of magnitude. The population of these novel nanoparticles is only about 0.1-1% in standard colloid preparations but can be enriched to about 10-15% by size-selective fractionation. This new class of nanoparticles could find potential uses in optoelectronic devices, ultrasensitive chemical sensors, and singlemolecule detection.
Introduction Optical and electrical transport studies at the level of single nanoparticles have provided new and novel insight into the sizedependent properties of nanostructured materials.1-3 These studies overcome the problem of inhomogeneous broadening caused by variations in the particle size, shape, and surface defects.4 By using surface-enhanced Raman scattering (SERS), we have recently discovered a new class of Ag nanoparticles that are able to enhance the efficiencies of surface optical processes by 14-15 orders of magnitude.5 This enormous enhancement allows optical detection and spectroscopy of single adsorbate molecules at room temperature. When excited with a continuous-wave laser, these metal nanoparticles also display a discontinuous photon-emission behavior, similar to intermittent photoluminescence in single CdSe quantum dots.6 However, the population of these novel particles is only about 0.1-1% in Ag colloids; that is, only 1 out of about 100-1000 colloidal particles is optically active. This low concentration represents a significant problem in developing these nanoparticles for potential uses in optoelectronic devices, ultrasensitive chemical sensors, and single-molecule detection. In this Letter, we demonstrate that the population of novel nanoparticles in a heterogeneous colloid can be increased or enriched by size-selective fractionation. The experimental design uses nuclear track-etched polycarbonate membranes (Nucleopores) to separate a heterogeneous colloid into different size fractions. These membranes have relatively uniform pore sizes (∼5% root mean square) and have been used by Martin’s group in nanomaterials synthesis.7 The rationale for this procedure comes from correlated optical and topographic studies of single particles with an integrated optical and atomic force microscope (AFM). The results show that optical enhancement is strongly correlated with particle size and the highly active particles fall in a narrow size range of about 90-100 nm.5 Experimental Section Silver colloids were prepared by the standard citrate-reduction procedure of Lee and Meisel.8 Morphological measurement by * To whom correspondence should be addressed. Email:
[email protected].
Figure 1. Enrichment of optically active nanoparticles by size-selective filtration: (A) schematic diagram of the Nucleopore filtration apparatus; (B) collection and resuspension of fractionated colloidal particles.
transmission electron microscopy (TEM) showed that the colloids were a mixture of heterogeneous particles with an average particle size of 35-50 nm and a typical concentration of ∼1011 particles/mL. These values are in agreement with previous TEM studies of Ag colloids.9 The colloids were separated into four fractions by sequential filtration using 100nm, 80-nm, and 50-nm pore size membranes (Figure 1). After each step, the retained colloidal particles were resuspended by sonicating the membrane in 0.01% sodium citrate solution. The first fraction contained large particles and aggregates above 100 nm, the second fraction contained relatively large particles
S1089-5647(97)03403-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/15/1998
494 J. Phys. Chem. B, Vol. 102, No. 3, 1998
Letters
Figure 2. Screening for optically active nanoparticles in different Ag colloid fractions. A large number of colloidal particles were immobilized on a glass surface and were screened by wide-field epi-illumination. A high-performance optical filter was used to pass the Raman-scattered photons (Stokes-shifted) and to reject the scattered laser light. The images were recorded with a cooled, back-thinned CCD camera. The particle densities in (C) and (D) were 2-3 times higher than those in (A) and (B). Note that only the particles with high enhancement efficiencies were observed. Laser wavelength is 514.5 nm; laser power is 10 mW; data integration time is 10 s.
between 80 and 100 nm, the third fraction consisted of mediumsized particles between 50 and 80 nm, and the last fraction (supernatant) contained small particles with diameters below 50 nm. Double-filtration experiments showed that membrane clogging was not a problem at these pore sizes and that essentially no particles were retained on the membrane during the second filtration step. The particle sizes in each fraction were checked and confirmed by tapping-mode AFM. It should be noted, however, that the silver particles are faceted nanocrystals and do not have regular shapes. Their sizes are best described by the average values of their long and short axes. When the Ag colloids are filtered with uniform and straight pores, the short axial dimension determines whether a particle goes through the membrane. Indeed, we have occasionally
observed rod-shaped particles as long as 500 nm in the 80100-nm fraction. To search for optically active particles, aliquots of the fractionated colloids were applied to chemically modified surfaces (polylysine-coated slides). The particles were quickly dispersed by liquid flow and were immobilized on the surface by electrostatic interactions between the negative charges on the particles and the positive charges on the surface. Because the resolution of far-field optical microscopy is diffractionlimited, individual particles would not be resolved at high particle densities. This problem can be overcome by diluting the colloid concentration or reducing the sample volume. In this work we reduced the particle density to about 0.1-0.01 particle/µm2. At this level, the probability for finding two
Letters
J. Phys. Chem. B, Vol. 102, No. 3, 1998 495
Figure 3. Tapping-mode AFM images of fractionated silver nanoparticles. The AFM and optical images were obtained from similar samples but did not cover the same areas on the glass slide, and different scale bars were used.
particles within the diffraction limit (half of the light wavelength or ∼250 nm at 514-nm laser excitation) is very small, while 6-60 million particles can still be screened on a standard microscope slide (20 mm × 30 mm). Results and Discussion Figure 2 shows the optical screening results of silver particles that are incubated with 1.0-mM NaCl and 2.0-nM rhodamine 6G for about 2 h at room temperature. The amount of free R6G (not adsorbed on particles) is determined to be ∼20% by centrifugation of the silver particles and fluorescence measurement of the supernatant solution. We estimate that the average number of analyte molecules per particle is about 10-40. This number (analyte-to-particle ratio) can vary by a factor of 4 in different fractions because a 100-nm particle has four times the surface area of a 50-nm particle. An important finding is that most SERS-active particles are concentrated in the 80-100nm fraction. Correlated optical and AFM measurements indicate that these particles are mostly spatially isolated, single nanoparticles. At the low electrolyte concentration used, extensive colloid aggregation does not occur.9 No active particles are detected in the 100-nm fraction. This optical
activity could arise from particle aggregates or from rod-shaped and other single particles that did not pass through the 100-nm membrane. Further studies are needed to answer this question. Size-selective fractionation also provides an opportunity to examine the factors responsible for efficient optical enhancement and to study the mechanisms of SERS.10 Two experimental findings are worth noting: (a) approximately 10-15 particles out of 100 (i.e., 10-15%) are optically active in the 80-100nm fraction, and (b) these particles exhibit large variations in signal intensities. With an integrated optical and atomic force microscope, we are able to directly examine the size and shape of the active and inactive particles (Figure 3). The results confirmed the particle sizes in different fractions and revealed that the optically active particles have a distinctly faceted shape. TEM imaging showed further structural details of these nanocrystals (data not shown). At the present, however, it is not clear how the faceted shapes are related to surface-active sites, which could be short-range chemical sites (adatoms, clusters, and defects) or long-range physical sites (sharp edges and points).11,12 To be certain that the observed signals arise from Raman scattering and not from fluorescence or other optical processes, we have studied the wavelength-resolved spectra of rhodamine 6G (a fluorescent dye) and 3-hydroxykynurenine (a nonfluorescent biomolecule) (Figure 4). The spectra were obtained by using a single-stage spectrograph/CCD detector attached to an
496 J. Phys. Chem. B, Vol. 102, No. 3, 1998
Letters of colloidal nanoparticles. A new class of metal nanoparticles has been identified and enriched from heterogeneous Ag colloid preparations. In the enriched colloid, about 10-15% of the particles are SERS-active. Other techniques such as sizeexclusion chromatography16 and size-selective precipitation17 could lead to further enrichment, but these methods are unlikely to yield 100% active colloids (i.e., all particles are active) because of particle shape variations. It is thus important to develop synthetic routes that could produce more uniform colloidal particles. One approach is perhaps to use small gold particles (2-10-nm diameter and nearly monodisperse)18 as “seeds” to prepare Ag colloidal particles in the size range of 80-100 nm. Such highly active nanoparticles should be useful in single-molecule spectroscopy, ultrasensitive chemical analysis, and nanomaterials design.19,20 The methods and instrumentation of this work could also be extended to search for novel size-dependent properties in other nanostructures such as CdS and CdSe quantum dots.4 Acknowledgment. We thank Warren Chan for technical help in tapping-mode atomic force microscopy and Adam Reuter, Dongsong Wang, and John Krug for colloid filtration. This work was supported by the National Science Foundation (CHE9610254), the Beckman Foundation Young Investigator Program, and the Petroleum Research Fund (PRF #32231-AC5). References and Notes
Figure 4. Surface-enhanced Raman spectra of 3-hydroxykynurenine (A) and rhodamine 6G (B) adsorbed on single silver nanoparticles. Laser wavelength is 514.5 nm; excitation intensity is 2.5 µW; data integration time is 10 s; R6G concentration is 2.0 nM; hydroxykynurenine concentration is 1.0 µM.
inverted microscope. It should be pointed out that resonance enhancement contributes to the R6G Raman spectra at 514.5nm excitation, but the large enhancement factors observed with near-infrared laser excitation indicate that surface enhancement is the dominant effect.13 Hydroxykynurenine has no optical absorption in the visible region, and its surface Raman spectrum does not contain a resonance component. Intense SERS signals are observed from this biomolecule only at fairly high concentrations (µM). We believe that this is caused mainly by the surface adsorption properties of hydroxykynurenine and not by the lack of resonance enhancement. Overall, the spectral frequencies observed in this study are similar to the resonance Raman and surface-enhanced Raman signals previously reported for these two molecules.9,14 Taken together, these results demonstrate that rare nanoparticles in a heterogeneous population can be detected and enriched by simple procedures. This naturally raises a fundamental question: have the size and shape libraries of silver nanoparticles been searched completely and the maximum enhancement achieved? Considering the large number (over 1 billion) of particles screened, we have most probably identified the most active Ag nanoparticles. On the basis of recent SERS studies of single R6G and crystal violet molecules,6,13 the intrinsic enhancement factors on such Ag nanoparticles are as large as 1014-1015. It is clear that surface plasmon excitation plays an important role, but current theories on SERS and the optical properties of small metal particles cannot explain these large enhancement factors.10-12,15 In conclusion, this work presents a rapid screening and enrichment strategy for exploring the size and shape libraries
(1) For optical studies of single quantum dots, see: (a) Gammon, D.; Brown, S. W.; Snow, E. S.; Kennedy, T. A.; Katzer, D. S.; Park, D. Science 1997, 277, 85-88. (b) Bockelmann, U.; Heller, W. Phys. ReV. B. 1997, 55, 4456-4468. (c) Hessman, D.; Castrillo, P.; Pistol, M.-E.; Pryor, C.; Samuelson, L. Appl. Phys. Lett. 1996, 69, 749-751. (d) Leon, R.; Petroff, P. M.; Leonard, D.; Fafard, S. Science 1995, 267, 1966-1968. (2) For electrical transport studies of single quantum dots, see: (a) Ralph, D. C.; Black, C. T.; Tinkham, M. Phys. ReV. Lett. 1995, 74, 32413244. (b) Klein, D. L.; McEuen, P. L.; Bowen Katari, J. E.; Roth, R.; Alivisatos, A. P. Appl. Phys. Lett. 1996, 68, 2574-2576. (c) Andres, R. P.; Bein, T.; Dorogi, M.; Feng, S.; Henderson, J. I.; Kubiak, C. P.; Mahoney, W.; Osifchin, R. G.; Reifenberger, R. Science 1996, 272, 1323-1325. (3) For electrical transport studies of single carbon nanotubes, see: (a) Bockrath, M.; Cobden, D. H.; McEuen, P. L.; Chopra, N. G.; Zettl, A.; Thess, A.; Smalley, R. E. Science 1997, 275, 1922-1925. (b) Tans, S. J.; Devoret, M. H.; Dai, H.; Thess, A.; Smalley, R. E.; Geerligs, L. J.; Dekker: C. Nature 1997, 386, 474-477. (c) Dai, H.; Wong, E. W.; Lieber, C. M. Science 1996, 272, 523-526. (4) For recent reviews, see: Alivisatos, A. P. Science 1996, 271, 933937; J. Phys. Chem. 1996, 100, 13226-13239. (5) Nie, S.; Emory, S. R. Science 1997, 275, 1102-1106. (6) For intermittent photon emission in single quantum dots, see: (a) Nirmal, M.; Dabbousi, B. O.; Bawendi, M. G.; Macklin, J. J.; Trautman, J. K.; Harris, T. D.; Brus, L. E. Nature 1996, 383, 802-804. (b) Blanton, S. A.; Hines, M. A.; Guyot-Sionnest, P. Appl. Phys. Lett. 1996, 69, 39053907. (c) Empedocles, S. A.; Norris, D. J.; Bawendi, M. G. Phys. ReV. Lett. 1996, 77, 3873-3876. (7) Martin, C. R. Science 1994, 266, 1961-1966; Acc. Chem. Res. 1995, 28, 61-68. (8) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391-3395. (9) Hildebrandt, P.; Stockburger, M. J. Phys. Chem. 1984, 88, 59355844. (10) Moskovits, M. ReV. Mod. Phys. 1985, 47, 783-826. (11) Schatz, G. C. Acc. Chem. Res. 1984, 17, 370-376. (12) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: Condens. Matter 1992, 4, 1143-1212. (13) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Phys. ReV. Lett. 1997, 78, 1667-1670. (14) Emory, S. R.; Nie, S. Anal. Chem. 1997, 69, 2631-2635. (15) Barber, P. W., Chang, R. K., Eds.; Optical Effects Associated with Small Particles; World Scientific: Singapore, 1988. (16) Fischer, Ch.-H.; Lilie, J.; Weller, H.; Katsikas, L.; Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1989, 93, 64-70. (17) Fischer, Ch.-H.; Weller, H.; Katsikas, L.; Henglein, A. Langmuir 1989, 5, 429-432. (18) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718-724.
Letters (19) 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 1995, 267, 1629-1632.
J. Phys. Chem. B, Vol. 102, No. 3, 1998 497 (20) (a) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607-609. (b) Elghanian, R.; Storhoff, J. J.; Mucic, R. C.; Letsinger, R. L.; Mirkin, C. A. Science 1997, 277, 1078-1081.
