Colloidal metal films as a substrate for surface-enhanced spectroscopy

9466

J. Phys. Chem. 1995, 99, 9466-9471

Colloidal Metal Films as a Substrate for Surface-Enhanced Spectroscopy George Chumanov, Konstantin Sokolov, Brian W. Gregory, and Therese M. Cotton* Ames Laboratory and the Department of Chemistry, Iowa State University, Ames, Iowa 50011 Received: October 31, 1994; In Final Form: January 26, 1995@

Colloidal films of gold and silver were prepared on glass or quartz slides. The slides were derivatized with (3-mercaptopropyl)trimethoxysilaneand subsequently reacted with aqueous metal colloids for variable time periods. The formation of the sulfur-metal bond provides a stable colloidal film on the surface. Because of the electrostatic interaction between individual particles, a semiregular structure is produced, as can be seen from electron micrographs. The unique property of the colloidal film is that they possess the optical properties of colloidal metals and the convenience of solid substrates. The effect of the dielectric constant of solvents on the optical frequencies, as well as the specific interaction of the solvent molecules with the metal on the plasmon resonances, was examined in detail. The colloidal films exhibit strong enhancement of Raman scattering and fluorescence emission from molecules adsorbed on the surface. Enhancement of fluorescence was observed for fluorescein-labeled molecules spaced 0-200 8, away from the surface. These substrates can be used in a number of analytical applications, such as surface-enhanced spectroscopies as well as for fundamental studies of plasmon resonances in small metal particles.

Introduction The application of surface-enhanced Raman spectroscopy (SERS) to a number of analytical and biophysical problems has stimulated a search for new surfaces for enhancing optical phenomena. In addition to commonly used electrochemically roughened Ag and Au surfaces, island films, and colloidal suspensions of these metals,’ a large number of new solid substrates have been developed, primarily for enhanced Raman scattering and aimed toward higher enhancement and greater reproducibility. It is well known that appreciable enhancement of Raman scattering requires a roughened metal surface.* Chemical etching of smooth silver surfaces by halide ions,3 a mixture of Cr03-H2S04: and nitric acid5 has been successfully used to produce SERS-active substrates. The requisite roughness can also be achieved by deposition of the metal on an inherently rough substrate, such as alumina, titanium dioxide, filter paper,9 and latex beads.l03’ and fumed silica In other systems, alumina and titanium dioxide particles were deposited from colloidal suspensions onto glass slides prior to metal depo~ition.’~,’~ Polymer membra ne^'^ and zeolites15 were used as a matrix for Ag particles formed by in situ chemical reduction. More exotic substrates include vacuum-deposited Ag on Si02 prolate postsI6 or through a nucleopore membrane mask,” and flake silver paint which consists of platelike particles.Is All of these methods produce “surfaces” that have the appropriate roughness for producing enhanced Raman scattering. Colloidal metals play a special role in surface-enhanced spectroscopies because they exhibit strong enhancement and are relatively easy to prepare. They also provide the possibility to study the fundamental optical properties of small conducting particles and to correlate these properties with the enhancement phenomenon. Chemical reduction methods are frequently used to prepare colloidal metals; these have been reviewed by Turkevich et aZ.I9 Recently, a new method was introduced for colloid preparation based on ablation of metals by high peak power laser pulses.20

* Author to whom correspondence should be addressed. @

Abstract published in Advance ACS Abstracts, May 15, 1995.

Although a suspension of colloidal metals is a “convenient” system for conducting optical measurements, generally it is an unstable system in comparison to solid substrates because of the tendency for these particles to aggregate spontaneously upon addition of analytes. At the same time, aggregation appears to be crucial for enhancement and is the main source of irreproducibility in the magnitude of the signal. In this paper, colloidal metal films are introduced as a hybrid system which combines the advantages of colloidal suspensions and the stability of solid substrates. These films are prepared by attaching the silver and gold (other metals are possible) particles from a colloidal suspension to a glass or quartz slide derivatized with (3mercaptopropy1)trimethoxysilane (MPS). Adsorption is accomplished through formation of metal-sulfur bonds at the surface. This system exhibits enhancement of Raman scattering comparable to electrochemically roughened surfaces, as well as enhanced fluorescence. Colloidal metal films provide a unique opportunity to control the aggregation state prior to experiments and preserve this state irrespective of the analyte, solvent, etc. throughout all measurements.

Experimental Section Colloid Preparation. Silver and gold colloids were prepared by citrate reduction according to previously described procedures.*’ Laser ablation of metal into aqueous solution20 was also used for preparation of colloidal suspensions. MPS-Derivatization of Glass Slides. Silanization of glass slides with (3-mercaptopropyl)trimethoxysilane(MPS, United Chemical) in 2-propanol (Fisher) was performed according to a previously published procedure.22 Prior to silanization, the slides were cleaned ovemight in Na2Cr207 (Fisher)/concentrated H2S04 (Fisher) solution. All chemicals were used as received. Monolayer Deposition. Monolayers of 11-mercaptoundecan01 (MUD) were self-assembled onto Ag and Au substrates according to established protocol.23 Self-assembly was accomplished by immersion of the substrates in 1 mM MUD/ methanolic solutions for ’12 h, after which they were thoroughly rinsed with HPLC grade methanol (Fisher). Monolayers of octadecanoic acid (ODA, Applied Science) and mixtures of N-( 5-fluoresceinthiocarbamoy1)-1,2-dihexade-

0022-3654/95/2099-9466$09.00/0 0 1995 American Chemical Society

Substrate for Surface-Enhanced Spectroscopy

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canoyl-sn-glycero-3-phosphoethanolamine, diethylammonium Electron micrographs of gold and silver colloidal films are salt (FDPPE, Molecular Probes), with 1,2-dihexadecanoyl-snshown in Figure 1. High density Au and low density Ag films glycero-3-phosphoethanolamine (DPPE, Sigma, 99%) were were obtained after exposure of slides for several days to prepared at the gadliquid interface in a thermostated, solid colloidal suspensions of approximately the same concentration. Teflon Langmuir trough placed within a nitrogen-purged box. “High” and “low” density is defined here relative to the mean A solution of 1 mM MgS04 in ultrapure water (Millipore Millidistance between particles. When the latter becomes smaller Q, nominal resistivity of 18 MBScm) was used as the subphase than the average diameter of the particles, the film is designated for all preparations. The subphase temperature was maintained a high-density film. For the converse, it is designated a lowat 21 & 1 “C. The ODA and FDPPE/DPPE (1:3 and 1:24, mol/ density film. Remarkably, in spite of the fact that the lowmol) spreading solutions were prepared with either pure CHCl3 density film has space for more particles to adsorb on the or CHC13:CH30H (1: 1, v/v) and were typically 1.2 and 1.4 mg/ surface, these sites are not filled, even with prolonged exposure mL in lipid concentration, respectively. After an appropriate (several days) of the film to the colloidal suspension. Higher volume of solution was spread on the subphase surface, an density films, however, can be obtained by exposure to higher equilibration period of 10-15 min was allowed for solvent concentrations of the initial colloidal suspension. For example, evaporation before compression was begun. The monolayer was exposure of slides to a colloidal suspension of Ag with an optical then compressed at a rate of 5 cm2/min to the desired transfer density of ca. 30 for several days will yield a film with an optical surface pressure (IItransfer), after which another 10-15 min density of 1.O- 1.2. A decrease i n the optical density of the equilibration period was allowed. Surface pressures were colloidal suspension to a value of ca. 3 produces a film with an recorded by differential weight measurements using a filter paper optical density of ca. 0.4 following the same exposure time. Wilhelmy plate suspended from a linear variable differential Such nonlinear behavior can be explained by invoking repulsive transducer (LVDT). After equilibration at IItransfer, the monointeraction (presumably electrostatic) between particles in the layer was transferred at constant pressure (IItransfer & 0.2 mN/ suspension. Interparticle repulsion may also be responsible for m) at a linear transfer rate of 1.5 mm/min. For ODA and the low-density film formation in which fairly regular distances FDPPE/DPPE monolayers, IIItransfer values were 21 f 1 and 36 are observed between particles. Preliminary data show that & 1 mN/m, respectively. Transfer ratios for ODA and FDPPE/ specific adsorption of different molecules on the surface of the DPPE monolayers were typically 1.1 f 0.1. ODA depositions were Z-type, whereas DPPE/FDPPE depositions were Y - t y ~ e . ~ ~ colloids can be used to change the interaction between particles and, consequently, the density of the colloidal film. Instrumentation. UV-vis spectra were obtained with a Lambda 6 (Perkin-Elmer) spectrophotometer in the normal Gold colloidal films are extremely stable while stored in water incident geometry. For measurements in different solvents, the or organic solvents. No significant changes in extinction spectra slides were placed into a standard 1-cm quartz cuvette. A slide were observed after storage of these films in water for several without the colloidal film was used as a reference. months. In the case of Ag colloidal films, storage in water for Raman spectra were obtained with the 514.5 nm excitation the same period of time resulted in a decrease and a red shift in line from an Ar+ laser (Coherent, Innova 200). The scattered the extinction maximum. The decrease is due to the loss of light was collected by a camera lens (fll.2) in a backscattering particles from the surface (20% following the first month of geometry. The spectra were analyzed with a triple spectrometer storage), whereas the red-shift may be the result of adsorption (Spex, Triplemate 1877) at a resolution of 6 cm-’ equipped of impurities on the colloidal film. A similar shift was observed with a liquid nitrogen CCD detector (LN-1152, Princeton when monolayers of fatty acids were transferred to the film and Instruments). Indene was used to calibrate the spectra. was accompanied by a decrease in extinction of approximately Fluorescence measurements were measured with a com10%. These changes appeared to be reversible upon removal mercial fluorometer (FluoroMax, Spex) as well as with the of the monolayer with chlorofondmethanol (50/50). Storage Raman equipment described above. of the films in 2-propanol for 30 min was also found to decrease Electron microscopy was performed in the reflection mode the extinction by about 4%. In contrast, no changes were using a JEOL 1200EX scanning transmission electron microobserved under the same conditions when methanol, benzene, scope. A platinudpalladium (80:20) film of approximately or hexane was used. Gold films were not affected by any of 100-8, thickness was deposited on the specimens using a Polaron the above solvents. However, both gold and silver films are E5100 sputter coater. stable while stored in colloidal solutions. Results and Discussion The extinction spectra of gold and silver colloidal films in Characterization of Colloidal Metal Films. Colloidal films different media are shown in Figures 2 and 3. Remarkably, of gold and silver were formed by immersing glass slides the spectra for low- and high-density films in water are very derivatized with MPS into a colloidal suspension. The adsorpsimilar to those of the initial colloidal suspension used in their tion of metal particles on the slide was monitored by W - v i s preparation. This suggests that the particles do not strongly spectroscopy. The adsorption kinetics can be divided superfiinteract with each other under irradiation even though the mean cially into fast and slow regimes. In the fast regime, the distance between particles in the high-density film (on the order concentration of particles on the surface achieves equilibrium of 100 8,) is near in size to their diameter. Excitation of the in less than 1 h based on the maximum in absorbance. The plasmon resonance, or oscillation of the electron density, which actual value of the absorbance depends on the initial concentraoccurs under irradiation does not apparently induce an aption of the colloid. If the slides are kept in the highly preciable dipole moment in the particles. If such were the case, concentrated colloidal suspension (optical density 20 or greater) strong coupling between induced dipole moments of the for a time period of weekdmonths, new features occur in the individual particles would cause changes in the extinction extinction spectrum mainly to the red of the main plasmon spectra, such as a shift in the frequency of the maximum in the resonance. These features are similar to those observed when plasmon band (analogous to aggregation in molecular systems). a colloidal suspension aggregates and can readily be assigned Because the colloidal films are highly stable in different to formation of complexes in which particles clump together (hereafter referred to as aggregated colloidal films). solvents, they provide a convenient system for studying the

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9468 J. Phys. Chem., Vol. 99, No. 23, I995

Figure 1. Electron micrographs of (top left) high-density and (top right) aggregated Au colloidal films and (bottom) low-density Ag colloidal film.

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Figure 2. Extinction spectra of Ag low-density colloidal film

Figure 3. Extinction spectra of Au high-density colloidal film

submersed in different solvents: (a) benzene; (b) hexane; (c) 2-propanol (dotted line); (d) water; (e) air.

submersed in different solvents: (a) benzene; (b) hexane; (c) 2-propanol (dotted line); (d) water; (e) air.

effect of the dielectric environment on the plasmon resonance of the particle. In this preliminary study, extinction spectra were obtained for the Ag and Au colloidal films in benzene, hexane, 2-propanol, water, and air (Figures 2 and 3). To our knowledge, these are the first experimentally determined optical spectra of colloidal metals in organic solvents. Liao et aZ.25 showed experimentally that the excitation profile for Raman enhancement shifts to the red as the dielectric constant of the medium increases. Barber et a1.26performed electrodynamic calculations on silver spheroids and predicted the optical spectra for prolate spheroids in air, water, and cyclohexane. However, the direct

experimental determination of optical spectra of colloids has been impossible in the past because of the instability of metal colloidal suspensions in organic solvents. Furthermore, this approach is being extended to include the effect of strong chemical interactions (Le., charge donation or withdrawal) of adsorbed molecules on the plasmon resonance of a small metal particle. It was found for both Ag and Au films that the position of the peak of the plasmon resonance red shifts as the refractive index at the optical frequency of the surrounding medium increases. For small particles in the Rayleigh scattering regime, the extinction spectrum is primarily determined by absorption

Substrate for Surface-Enhanced Spectroscopy

J. Phys. Chem., Vol. 99, No. 23, I995 9469

TABLE 1: Selected Properties of Different Solvents and AiP a B n* n 0 0.00

benzene hexane 2-propanol water air

0.76 1.17

0.1 0.00 0.84 0.47

0

0

0.59

-0.04 0.48 1.09

0

10 I\

?!!

h

1.498 1.372 1.375 1.333 1.0003

a,hydrogen-bond donation ability; /3, hydrogen-bond acceptance ability (electron donation ability); n*, polarizability; n, refractive index at optical frequency.

of the particles given by .r

+

where E = €1 i ~ is 2 the dielectric constant of the metal at the optical frequency relative to that of the surrounding medium, 1 is the wavelength in that medium, and a is the radius of the particles.27 Thus, the position, intensity, and half width of the absorption band depend on the dielectric constant of the medium. The different characteristics of the solvents used in this study are shown in Table 1. Note that the polarizability and hydrogenbond donatiodacceptance ability are significantly different for hexane and 2-propanol. However, since their refractive indices are practically the same, the extinction spectra in these solvents are identical (Figures 2 and 3). SERS of Self-Assembled Monolayers. Silver and gold colloidal films were used as substrates for SERS. A puzzling observation is that no SERS spectra could be obtained from the underlying MPS-coupling layer. In order to detect SERS of this molecule, the Ag film was exposed to a solution of MPS. The greatest enhancement of Raman scattering of self-assembled monolayers, which was comparable to that on electrochemically roughened electrodes, was observed on aggregated colloidal films. In contrast, high-density films exhibited very weak enhancement (close to the detection limit of the instrument used in this study). No SERS spectra were observed from these monolayers on low-density colloidal films. Moreover, spectra were observed only after exposure to the solution for about 24 h, even though the surface became completely hydrophobic after exposure for 1 h. A possible explanation is that the molecules must penetrate into the spaces between aggregated colloidal particles in order to produce strong SERS. It is known that the strongest enhancement comes from molecules situated between particles.28 A SERS spectrum of MPS chemisorbed on top of an aggregated colloidal film is shown in Figure 4A. The spectrum (Figure 4A) is in qualitative agreement with that obtained for MPS on a roughened Ag electrode.29 The peaks at 704 and 628 cm-' have previously been assigned to the trans and gauche C-S stretches, respectively (i.e., ~ ( c - s )and ~ Y(c-s)G).30-32 The nearly equal relative intensities of these two bands indicate that there is no preferred conformation between headgroup and alkyl tail. This further suggests that the chemisorbed film may be quite amorphous, exhibiting no long-range lateral order as would be required for a truly self-assembled system. In contrast, a self-assembled monolayer (SAM) of MUD on an aggregated silver colloidal film is remarkably well ordered, as evidenced by the SERS spectrum in Figure 4B. For comparison, a self-assembled monolayer of MUD on a roughened, polycrystalline Ag electrode is shown in Figure 4C. Peak positions and relative intensities between the two attest to the suitability of these colloid-coated slides as standard SERS substrates. It is also apparent from the large

* h

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Wavenumbers (cm-1) Figure 4. SERS spectra of (A) MPS and (B) MUD chemisorbed on aggregated Ag colloidal film and (C) MUD chemisorbed on roughened Ag electrode. Spectra were obtained with 514.5 nm excitation. The laser power at the sample was ca. 5 mW.

Y(C-S)T/V(C-S)G relative intensity ratio that the chains exist primarily in a trans conformation near the headgroup. The virtual exclusion of the gauche mode implies a high degree of order in this system; this was confirmed by the Raman spectra of bulk solid MUD (not shown), which completely lacked the Y(C-S)G vibrational band. In Raman spectra of lipid-based systems, the C-C stretching (1150-1000 cm-I) and CH2 deformation (1500-1400 cm-I) regions are the most useful for determining both overall chain conformation and lateral packing order.23 The peaks at -1 108 and 1064 cm-' in parts B and C of Figure 4 have previously been assigned to the symmetric and asymmetric all-trans C-C stretches (Le., Y,(C--C)T and Y,(C-C)T), r e s p e ~ t i v e l y . ~The ~.~' peak positions and line widths, compared with that of the bulk solid (not shown), are consistent with the presence of fully extended alkyl chains. In addition, a splitting of the methylene deformation mode d(CH2) (due to crystal field effect^^^-^^) into two distinct, though overlapping, bands is observed at 1453 and 1435 cm-I. The origin of this effect, coupled with the evidence for fully extended, all-trans hydrocarbon chains, indicates that the MUD SAM exists within a crystalline-like environment. The self-assembly of MUD onto an aggregated Au colloidal film is shown in Figure 5A. A SERS spectrum of MUD selfassembled on a roughened Au electrode is displayed in Figure 5B for comparison. With the exception of a few intensity differences, the spectra for the two are identical. The d(CH2) mode in these spectra is split into two sharp, well-resolved bands

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9470 J. Phys. Chem., Vol. 99, No. 23, 1995

The fluorescence of a fluorescein-labeled phospholipid (FDPPE)/phospholipid (DPPE) mixed monolayer transferred by the Langmuir-Blodgett (L-B) technique to a colloidal Ag film was measured as a function of the number of spacer layers of X 5 ODA. In order to account for self-quenching of fluorescence between molecules within a single monolayer, mixed monolayers were prepared at mole ratios 1:3 and 1:24 for labeled and unlabeled phospholipids, respectively. All spectra obtained from Ag colloidal films were compared to those obtained from a multilayer structure (three layers of ODA followed by a mixed monolayer) on a bare glass slide as a reference. In all cases of enhanced and nonenhanced fluorescence, the signal from the 1:24 mixed monolayer was only approximately 2 times less than that of the 1:3 monolayer, even though the fluorophore concentration in the former was nearly 6 times lower than that 0 8 in the latter. This suggests that self-quenching occurs in monolayers with a high concentration of FDPPE, but it does not affect the fluorescence enhancement phenomenon. $ All fluorescence measurements were performed using both t. 0 2 the Raman equipment and standard fluorometer described in the Experimental Section. Both instruments yielded identical results. Emission spectra of the 1:3 mixed monolayer on a Ag colloidal film are shown in Figure 6. These spectra were n R obtained using 488.0 nm laser excitation. The same experiments were performed using 457.9 nm excitation. Since the fluorest 0 N 0, cence maximum of FDPPE occurs at ca. 520 nm and the Raman m m t spectrum appears to be centered around 485 nm (with excitation at 457.9 nm, data not shown), the contribution of these two phenomena to the observed signal can be distinguished. From a comparison of spectra obtained with 488.0 and 457.9 nm 1400 1200 1000 800 600 excitation, the conclusion was made that the background in the spectra of Figure 6 is mainly associated with fluorescence, not Wavenumbers (cm-1) with SERS. The SERS phenomenon is known to be acFigure 5. SERS spectra of MUD chemisorbed on (A) aggregated Au companied by an elevated background. The most remarkable colloidal film and (B) roughened Au electrode. Spectra were obtained feature is that the strongest enhancement of both Raman and with 647.1 nm excitation. The laser power at the sample was ca. 10 fluorescence was observed when the monolayer was deposited mW. directly on the colloidal film (Figure 6A). The surface of the colloidal film is hydrophilic, and therefore, the phospholipid at -1453 and -1428 cm-’ and indicates highly ordered, L-B monolayers were deposited with the headgroups oriented orthorhombic subcell acyl chain packing in the monolayer. In toward the surface. In this case, the fluorescence from FDPPE addition, the appearance of the Y,(C-C)T mode at 1107 cm-’ is expected to be quenched by the metal. In order to explain (compare to parts B and C of Figure 4) demonstrates that acyl the observed enhancement of both Raman and fluorescence, it chain configuration within the MUD S A M is primarily all-trans. is proposed that two different populations of molecules give The “forest” of peaks observed in these spectra, however, is contributions to these signals. Enhanced Raman originates from unusual in comparison both with the SERS spectra taken from the labeled phospholipids adsorbed directly on the Ag particles, the Ag colloidal film (parts B and C of Figures 4) and with as well as from the molecules situated in the space between the previously published ~ p e c t r a . ~A ~more ~ ~ detailed ’ ~ ~ ~ analysis particles where the local field enhancement is expected to be of these spectra, and of related alkanethiols, on Au colloidal greatest.40 The fluorescence signal arises mainly from the films is currently in progress. population of molecules in the region between the particles Enhancement of Fluorescence. Silver colloidal films were where fluorescence quenching is expected to be the least.40In also tested for enhancement of fluorescence. It is known that the deposition process, the monolayers may bridge the particles fluorescence is quenched near a metal surface due to nonraor possibly collapse within the space between the particles, but diative energy transfer from the excited state of the molecule in either case their location would comply with the above to the metal. On the other hand, excitation of the plasmon explanation. resonances in the metal produces enhancement in the local field It can be clearly seen from Figure 6 that both the fluorescence near the surface which can lead to enhancement of different and Raman enhancement decrease as the thickness of the spacer optical phenomena including fluorescence. Extensive literature layer increases. All spectra are displayed on the same scale so exists describing the dependence of fluorescence quenching on that the relative intensities are preserved. On proceeding from the distance between molecules and metal surface^.^' The “standard” approach for obtaining fluorescence e n h a n ~ e m e n t ~ ~ ~zero ~ ~to one spacer layer (ca. 50 A), the Raman signal decreases more than 20 times, whereas the fluorescence decreases only is to use a spacer layer between the fluorescent molecule and by a factor of approximately 2. The enhanced fluorescence and the metal, thereby compromising the fluorescence quenching Raman scattering which can be observed with spacer layers from effect and the enhancement of the local field which decays with one to seven are mainly due to enhancement of the local field. distance depending on the morphology of surface. 0

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r

9)

Substrate for Surface-Enhanced Spectroscopy

J. Phys. Chem., Vol. 99, No. 23, 1995 9471 Acknowledgment. Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. This article was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences. References and Notes

e f

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530

520

51 0

Wavelength (nm) Figure 6. Emission spectra of mixed monolayers of fluorescein-labeled phospholipid (FDPPE)/phospholipid (DPPE) with mole ratioo 1:3 for different numbers of octadecanoic acid spacer layers (25 A thick) transferred by the L-B technique to aggregated Ag colloidal film: (a) mixed monolayer directly transferred on colloidal film; (b) one spacer layer; (c) three spacer layers; (d) five spacer layers; (e) seven spacer layers; (f)mixed monolayer with three spacer layers on bare glass slide (as a reference sample); (8) instrument background. Spectra were taken with 488.0 nm laser excitation and the Raman equipment for detection. The laser power at the sample was less than 0.3 mW.

Conclusions Colloidal metal films are new substrates which exhibit enhancement of both Raman scattering and fluorescence. These substrates offer advantages that combine those of other conventional types; their morphology can be defined in a manner similar to colloidal suspensions, and their “aggregation state” can be preserved during experimental procedures as in the case of solid substrates. In addition, they can easily be studied by transmission absorption spectroscopy in various environments such as different solvents, temperatures, etc. Because aggregation does not occur, the effect of dielectric environment, as well as chemisorption of different molecules, on the plasmon resonances of small metal particles can be independently investigated using these substrates. Such studies are currently in progress.

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