Anal. Chem. 1966, 58,3159-3163
3159
Chemical Procedure for Preparing Surface-Enhanced Raman Scattering Active Silver Films Fan N i a n d Therese M. Cotton* Department of Chemistry, University of Nebraska-Lincoln, A new procedure for preparlng substrates for surface-enhanced Raman scatterlng (SERS) analysis Is described. Conventional Tollen’s reagent was used to coat Ag onto frosted glass slides. This method Is extremely simple and results In highly reproducible and stable substrates. Optlmlzatlon of the experknental parameters (AgNO, concentratlon, solution temperature, and deposltlon time) was achleved by using 4,4’-blpyrldlne (BP) as the analyte. An enhancement factor of ca. 1 X lo5 was determined from a comparlson of the normal solutlon Raman intenstty of BP to Its SERS Intenstty. The detection llmlt achleved under the experimental conditions described here Is 1 X lo-’ for BP, with good llnearlty In the concentratlon vs. SERS Intensity plots over 3 orders of magnttude. The sllver-coated slides are conductlve and can be used as electrodes. Cyclk voltammograms of BP adsorbed onto the chemlcally coated glass slides were ldentlcal with those observed for BP adsorbed onto a Ag electrode. The results Illustrate the considerable potential of SERS as a routine method for trace organlc analysis.
Following the initial observations of surface-enhanced Raman scattering (SERS) on an electrochemically roughened Ag electrode surface (I-3), a number of different procedures have been developed to produce SERS-active substrates as summarized in recent reviews (4-6). The three most common types of substrates include electrodes, colloidal sols, and island films prepared by vacuum deposition. Under carefully controlled experimental conditions, each of these substrates exhibits sizable enhancement (6 orders of magnitude in many cases) in Raman or resonance Raman scattering from adsorbates at or near the surface. There is considerable variability in the magnitude of the enhancement in many of the previously described procedures, however, because of the large number of nonquantitated experimental parameters involved in preparing the substrate. If the SERS effect is to be useful as a general analytical technique, it is necessary to develop simple methods for preparing highly reproducible surfaces. Each of the commonly used methods suffers from at least one serious drawback in this respect, as discussed below. The preparation of a SERS-active silver electrode requires the use of an oxidation-reduction cycle (ORC), which produces a roughened silver surface. The ORC cycle influences the magnitude of the enhancement both by cleaning the surface and by producing surface roughness. A clean surface allows the adsorbate to interact strongly with the Ag surface. The strong interaction between the adsorbate and the surface is believed to give rise to a chemical enhancement mechanism. The surface roughness, on the other hand, gives rise to enhanced Raman scattering via an electromagnetic enhancement (EM) mechanism (4-6). The relative contribution of electromagnetic and chemical enhancement mechanisms to SERS is not completely defined a t present and undoubtedly varies for different substrate/adsorbates as well as different experimental procedures. Support for the chemical mechanism may be found in the observation that the ORC is not absolutely required for the observation of SERS (7,B). A factor of 10’ was estimated by Van Duyne (8)to result from surface roughness following an ORC for pyridine in 0.1 M KCl. The
Lincoln, Nebraska 68588-0304 experimental variables involved in the ORC have also been extensively scrutinized in recent years. Those factors which affect the enhancement factor include the number of ORC cycles, the amount of charge passed during the ORC, the concentration of solute, the electrode potential, and the nature of the electrolye (4-6, 8). In addition, illumination of the electrode during the ORC procedure has been shown recently to produce a significant increase in the enhancement ( S I I ) . Thus, there are many variables in the electrochemical experiment and each must be carefully controlled in order to result in a surface which gives highly reproducible enhancement factors for a given adsorbate. The use of silver sols for obtaining SERS has attracted considerable attention because of the apparent experimental simplicity. However, the simplicity is deceptive since there are a large number of experimental parameters associated with the sol preparation which are difficult to control. This includes especially the surface potential of the Ag particles. We have found that the presence of excess reductant (NaBH,) can result in the chemical reduction of the adsorbate (12). In addition, the silver particles tend to aggregate following the addition of an adsorbate. Aggregation leads to irreproducibility for two reasons. First, the sol absorption properties shift to the red when longitudinal plasmon modes are excited. Second, the larger particles begin to flocculate and settle out of solution. Thus, the Raman signal intensity is not constant with time. The preparation of rough silver films by vapor deposition results in much more reproducible and stable surfaces. Of particular interest here are two recent papers which demonstrate that vapor deposition of thick (2200 A) Ag films onto Teflon, polystyrene (13),or latex (14) spheres deposited on filter paper resulted in substrates which product strong SERS intensities for various organic adsorbates and good reproducibility between multiple runs. However, this method is time-consuming and requires access to a vacuum apparatus. There are some variables in methods utilizing vapor deposition techniques as well, including especially the thickness of the silver film, the temperature of the glass substrate during deposition of the silver, and the use of annealing procedures (4-6). In addition, unless the entire experiment is performed under vacuum, the film is exposed to the atmosphere following deposition. Even a brief exposure to the atmosphere results in contamination of the surface and the formation of an oxide layer. In the results that follow, a simple, highly reproducible chemical procedure for preparing SERS-active Ag substrates is described. The analytical utility of this method is demonstrated in a study of 4,4’-bipyridine (BP). A preliminary report has appeared recently (15) in which a somewhat different experimental procedure was used to prepare chemically reduced silver films. The results appear comparable to those reported here, although no detailed study of the experimental parameters was provided and the analytical potential was not evaluated. EXPERIMENTAL SECTION Chemicals and Experimental Procedures. 4,4’-Bipyridine was purchased from Aldrich Chemical Co. and was further purified by subliming in vacuum at ca. 84 “C. D-Glucose (monohydrate)
0003-2700/86/0358-3159$01.50/0 0 1986 American Chemical Society
3180
-
ANALYTICAL CHEMISTRY. VOL. 58. NO. 14. DECEMBER 1986
c b
Figure 1. Frame for holding slides. was purchased from J. T. Baker. Sodium sulfate (anhydrous) was obtained from Fisher Scientific Co. Sodium hydroxide pellets were purchased from Spectrum Chemical MFG Corp. Silver nitrate was purchased from D. F. Goldsmith Chemical & Metal Corp. The concentrated ",OH solution was obtained from J. T. Baker Chemical Co. The water used in these experiments was deionized and distilled in glass. The suhstrates used in this study were frosted glass microscope slides (Fisher Scientific Co.) coated with chemically reduced Ag films. Tollen's reagent was used to deposit the silver. Each of the experimental variables was studied in order to optimize the films for SERS measurements, and the data is discussed in the Results and Discussion section. The final procedure is described here. Tollen's reagent was prepared in a small beaker by adding about 10 drops of fresh 5% NaOH solution to 10 mL of 2-370 AgNO, solution, whereupon a dark-brown AgOH precipitate is farmed. This step is followed hy dropwise addition of concentrated ",OH, at which point the precipitate redissolves. The beaker containing the clear Tollen's reagent was then placed in an ice bath. The frosted slides (dimensions 1 cm X 0.3mm), which had been cleaned with nitric acid and distilled water, were placed into a Teflon frame, which could accommodate up to 15 slides (see Figure l),and placed into the Tollen's reagent. Three milliliters of 10% o-glucose was added to the solution with careful swirling to ensure mixing. The beaker was then removed from the ice bath and the solution allowed to reach room temperature. The beaker was placed into a water bath (55 "C) for 1 min followed by sonication for l min (Branson Sonicator, Model B22-4, 125 w). Finally, the silver-coated slides were rinsed several times with distilled water and again sonicated in distilled water for 30 s. The slides were then stored in distilled water for several hours prior to exposure to the adsorbate solution. By use of this procedure, slides were found to be stable in distilled water for up to 1 week. For the SERS experiments the slides were air-dried briefly and then dipped into 5-mm tuhes containing the adsorbate. The slides were in contact with the solution far at least 30 min. Instrumentation. An argon ion continuous wave (CW) laser (Innova 90-5)was used as an excitation source for the Raman spectrometry. The laser exciting line was 514.5 nm. An Anaspec 300-S premonochromator was used to remove the plasma lines, and spectra were acquired in the backscattering geometry. A Canon 55-mm fl1.2 camera lens was used to focus the Raman scattering light onto the slit of the monochromatorfspectrograph (Spex Tripemate 1877), which is coupled to a Model 1420 intensified SiPD detector (Princeton Applied Research Corp.). The Raman and SER spectra were collected and processed with an optical multichannel analyzer (OMA-2, Princeton Applied Research Corp.). R E S U L T S A N D DISCUSSION The experimental variables that were investigated in preparing the silver films from Tollen's reagent included the concentration of the AgNO, solution, the temperature at which the reduction was accomplished, and the time allowed for the reduction process. T h e effect of changing these variables on the SERS intensity of BP was determined and the results are summarized here. Figure 2 illustrates the effect of the AgN0, solution concentration on the intensity of the 1295-cm-' band of bipyridine. All other experimental parameters were held constant, including the temperature of the bath (55 "C), the deposition time (2 min), and the concentration of bipyridine solution (1
2
figurb 2. Normalized SERS intensity lor 1295-cm.' band of BP vs. concentration of AgNO, SOlUtiin used for chemical deposition.
1600 1400 ' 1200 1000 '
W.".""nlb*r
'
Shift cm-1
Figure 3. (A) Fbotomicropph taken by scanning elemon miCrOSCOpy (SEM) of the Ag film prepared from 2% AgNO, soiution. (0) Photcmicrograph taken by SEM of the Ag film prepared from 5 % AgNO solution. The bar equals 1.0 pm. (C) SERS spectrum of 2.17 X 10-3 M BP obtained from the silver film shown in A. (D) SERS spectrum of 2.32 X IO-' M BP obtained from the silver film shown in B. X M) that was used in the adsorption step. As the concentration of AgNO, was decreased from 5 to 2% a rapid increase in SERS intensity was observed. Lowering the concentration below 2% did not increase the signal further (below 1% the signal was found to decrease due to low coverage of the glass slide with Ag). Thus, a 2% solution of AgNO, was used in all subsequent studies. As might he expected, the concentration of AgNO, used in formation of the Ag film determines the thickness of the Ag deposited. The thickness, in turn, influences the morphology. As has also been observed for high-vacuum deposition procedures,chemically deposited Ag forms island-type structures at low coverage. Figure 3A shows a scanning electron photomicrograph of a film prepared from 2% AgNO, solution, and Figure 3B shows a film prepared from a 5% AgN03 solution. The respective SERS spectra for BP on these slides are shown in Figure 3, parts C and D. The particles in Figure 3A are fairly regular, with their diameters varying between 400 and 1500 A. Although there is some clumping or aggregation, the particles are distributed fairly regularly over the surface. In contrast, the particles in Figure 38 form elongated, irregular structures and are much larger in overall size. The effect of particle size and shape on the SERS enhancement factor has been predicted from EM and has been experimentally tested as well (4-6). Although the particles shown in Figure 3A are not the ideal spherical or spheroidal geometry, it is ohvious that they do conform better to the optimum shape predicted by EM theory. The SERS intensity for the film shown in Figure 3A was 10 times that of the thicker film in Figure 3B. Moreover, the results show that structure of the silver film can easily be varied hy changing the concentration of AgN03 solution. It was also found that greater enhance-
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
3161
1294
I
A
1600
I
.
.
1600
1
.
1400
1200
1000
W a v e n u m b e r S h i f t cm - 1
Flgure 4. SERS spectra of BP as a function of solution concentration: M, (C) 9.57 X lo-' M, (D) 9.57 (A) 9.57 X M, (B) 3.83 X X M, (F) 9.57 X lo-' M, (G) 9.57 X lo-' M. M, (E) 9.57 X
1400
1200
1000
W a v e n u m b e r Shift cm-1
Flgure 5. Spectra obtained with 514.5-nm excitation from BP for the following solution concentrations and laser powers: (A) 1 X lo-' M, on Ag film, 50 mW; (B) same solution and conditions as in D, but stored for 19 days; (C) 1.05 X lo-' M solution spectrum, 293 mW; (D) 1.08 X M on Ag film, 54 mW.
Laser power was 53 mW. ment occurred if frosted slides were coated with silver rather than smooth ones. Apparently, the roughness of the glass surface influences the Ag structure. The temperature of the bath used duhng the reduction of the Ag was also investigated. The optimum temperature for the concentration of AgN03 used here was determined to be 55 f 1 OC. Lower temperatures required a longer coating time, and higher temperatures produced a more aggregated surface. The reduction time was critical and was a function of the other experimental parameters. Two minutes was optimal for the production of the Ag films prepared a t 55 O C and from 2% AgNO3 Sonication was important since it resulted in a more homogeneous film. The use of the SERS substrates for the determination of adsorbate concentration in solution is illustrated in Figure 4. The spectra show the increase in signal intensity with increase in BP concentration from lo-' to M. The BP was adsorbed from the solutions by immersing the Ag-coated slides into the solutions for approximately 30 min. The signal intensity was found to increase approximately 2-fold from the initial value and remain constant after 30-min exposure to the BP solution. The spectra were recorded with the slide in contact with the B P solution using 53 mW of 514.5-nm excitation and an integration time of ca. 20 s. The detection limit (peak-to-peak signal-to-noise ratio = 2) for B P was 1 X low7M under the experimental conditions used here (Figure 5A). The presence of broad background peaks attributed previously to carbon contamination (16) precluded a further decrease in the detection limit, although background subtraction was found to enhance signal detection near M. Thus, longer integration times and background subtraction could conceivably reduce the detection limit to as low as M. Much lower detection limits should be achievable with adsorbates which are also in resonance with the laser excitation wavelength (surface-enhanced resonance Raman scattering, SERRS). Under these conditions the detection limits for M dithizone anion (17) and rhodamine 6G (18)were 1 X and 5 X M, respectively. The SERS signals were found to decrease slowly with time. Figure 5B shows the SERS of B P observed 19 days after the
>
cIn z W
c
z-
cn K
w
u)
W
2
c
Q: J W
K
10-7
10-6
10-5
10-3
10-2
CON C E NTR AT1 0 N ( M )
Figure 6. SERS intensity dependence on BP solution concentration.
slide was immersed in a solution of initial concentration of M. In spite of the fact that intensity of the 1.08 X 1295-cm-' Raman band has decreased to approximately 15% of its original value (Figure 5D), the spectrum is still distinguishable. An estimate of the Raman enhancement factor can be made from a comparison of the intensity of the 1295-cm-' band of B P adsorbed onto the silver film with the intensity of this band in solution for an equal number of molecules in the scattering volume. The BP solution spectrum could be observed a t a bulk concentration of lo-* M only by increasing the laser power 6-fold (Figure 5C). Even under these conditions, a much lower signal-to-noise ratio results as compared M solution. Assuming to the SERS spectrum for a 1 X monolayer coverage on the silver film, the SERS signal is about 5 orders of magnitude greater than that in solution. Figure 6 shows a plot of the normalized surface Raman intensity of BP as a dependence on the log of its concentration. The intensity of the 1295-cm-l band was normalized by the expression
where C, equals the counts for the peak height of the 1295-
3162
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14,DECEMBER 1986
l-----l
I
I
I
I 1 0 0 UAMP
1600
1400
1200
1000
i
I
I Wavenurnber Shift
i
Cm-l
M BP soFigure 7. (A) SERS spectrum obtained from 1.18 X lution. (5) SERS spectrum obtained from 3.7 X lo” M BP solution. VOLTS V S SCE
cm-’ band and C,, equals the counts for the valley closest to, and on either side of, the peak. Five measurements of Raman intensity for each slide were conducted by moving the laser spot vertically along the slide. Some variation in SERS intensities a t different points was seen, due to nonuniform roughness of the silver surface. The laser power is stable within f 3 mW, and this is not significant in comparison to the variation in the Ag surface roughness. The mean of five intensity measurements is plotted against the concentration in the Figure 6. The intensity vs. concentration plot exhibits three regions. The slope of the plot of signal intensities for BP adsorbed from low-concentration solutions ( 10-7-104 M) is small relative to M. The first region that for the region from to undoubtedly reflects the adsorption of BP with the aromatic rings lying flat on the Ag surface. A t higher concentrations, the ring orientation may assume a perpendicular geometry, with one of the nitrogens interacting directly with the Ag surface. Similar orientational changes in aromatic molecules adsorbed at metal electrodes have been observed by Soriaga and Hubbard from thin-layer absorption spectroscopy (19). Support for a change in BP orientation at ca. lo4 M may be obtained from a comparison of the surface spectra. The SERS selection rules predict strong enhancement of the ring breathing mode (1014 cm-’) for the perpendicular orientation, whereas these modes would be much weaker for the flat orientation (20). A comparison of parts A and B of Figure 7 (1.18x 10” and 3.7 x M spectra, respectively) indicates that this is indeed the case. The third region of this plot shows an even greater increase in signal intensity with concentration M and higher). This effect may be attributed to the formation of multilayers of BP. The total intensity now reflects SERS intensity for the first and subsequent layers. Even though the enhancement factor decreases monotonically for each successive layer, normal Raman scattering from the thick film also contributes to the spectrum. The spectra observed for BP adsorbed from to M solutions are similar to that obtained previously for B P adsorbed onto a Ag electrode at a potential of -500 mV vs. SSCE (saturated NaCl calomel electrode) following an ORC (21). The assignment for each band in the SERS spectrum of BP can be found elsewhere (21). The effect of different anions on SERS intensity has also been studied at Ag surfaces. It has been observed, for example, that C1- ion strongly enhances the SEW intensity for pyridine as well as other molecules. These effects have been explained in terms of a chemical-enhancement mechanism (4-6). In contrast to EM mechanisms, chemical-enhancement theories predict that SERS spectra should only be observed from the monolayer immediately adjacent to the metal surface. We
Figure 8. Cyclic voltammogram of 1.39 X Na2S04: scan rate, 10 mvls.
lo-* M BP in
0.258 M
have found that C1- and SO,’- anions do not affect the SERS intensity of BP. This may be due to the fact that BP is neutral and does not require counterion adsorption. In recent years “hyphenated” techniques have become increasingly powerful for solving analytical problems. The advantage of a spectroelectrochemical technique coupling SERS with CV or other electrochemical procedures lies in the inherent specificity of vibrational spectra as compared to absorption spectra. It is possible to identify chemical species that are formed coincidentally with charge transfer processes. This has been demonstrated in previous experiments with BP and water-soluble porphyrins (21,22). The chemically coated silver films described herein are conductive and can be used as electrodes. A cyclic voltammogram (CV) of BP obtained by using the Ag film as a working electrode is shown in Figure 8. A coil of flattened copper wire was sealed with Torr Seal to the top of the silver slide for electrical contact. No preconditioning of the slide was required prior to the electrochemical measurement. The CV in Figure 8 is identical to that obtained in a previous report which utilized a polycrystalline Ag wire as the working electrode (21). In addition, the silver film was extremely stable as an electrode, as shown by repeated cycling. Thus, other important advantages of the chemically reduced silver films over Ag sols are that they can be used as an electrode and their surface potential can be controlled. The chemically coated slides were also tested with other analytes including 2,4-dinitrophenol and p-nitrophenol, which exhibited strong SERS spectra. Two water-soluble porphyrins, tetrasodium meso-tetrakis(4-sulfanatopheny1)porphineand rneso-tetrakis(4-carboxyphenyl)porphine, were found to give rise to intense surface-enhanced resonance Raman when excited with laser wavelengths close to electronic transitions in these molecules. Thus, the types of compounds that undergo enhancement at the chemically deposited silver substrates will probably be similar to those studied on other SERS-active substrates. In summary, the results presented here have demonstrated the first analytical application of SERS using chemically coated silver films on frosted microscope slides. An enhancement factor of lo5 was observed using Tollen’s reagent for preparing silver-coated microscope slides. Scanning electron microscopy has shown that by careful control of the experimental parameters (concentration of AgN03 solution, temperature of the bath used in the reduction step, and reduction time) fairly regular Ag structures can be obtained which are uniformly deposited on the glass surface. The
Anal. Chern. 1986. 58.3163-3166
advantages of this substrate over other types include the simplicity of the procedure, the stability of the films (as compared to colloidal silver sols, for example), and the high reproducibility of the SERS spectra obtained with these surfaces. There is also minimal background interference and chemical reactions with the adsorbate since glucose is used as a reductant rather than NaBH, (as in the case of sols). Glucose does not adsorb onto the silver surface, and all of the chemicals used in the preparation of the film are washed from the surface prior to addition of the adsorbate solution. The results shown with BP illustrate the considerable potential of SERS as a very sensitive technique for trace analysis of organic compounds. ACKNOWLEDGMENT We appreciate the helpful suggestions and guidance of Qiao Feng during the initial stages of this research. Registry No. BP, 553-26-4; Ag, 7440-22-4. LITERATURE CITED Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. J . Chem. SOC., Chem. Commun. 1973, 80. Jeanmaire, D. L.; Van Duyne, R. P. J . Electroanal. Chem. 1977, 8 4 , 1. Albrecht. M. G.; Creighton, J. A. J . Am. Chem. SOC. 1977, 99, 5215. Chang, R. K.; Laube, R. L. CRC Crit. Rev. Solid State Mater. Sci. 1984, 12, 1. Chang, R. K.; Furtak, T. E. Surface EnhancedRaman Scattering; Plenum: New York, 1982. Moskovits, M. Rev. Mod. Phys. 1985, 57, 783.
3163
(7) Schultz, S. G.; Janik-Czachor, M.; Van Duyne, R. P. Surf. Sci. 1981, 104, 419. (8)Van Duyne, R. P. I n Chemical and Biochemical Applications of Lasers; Moore, c. B., Ed.; Academic Press: New York, 1979; Vol. 4, Chapter 5. (9) Barz, F.; Gordon, J. G., 11; Phllpott, M. R.; Weaver, M. J. Chem. Phys. Lett. 1982, 91, 291. (10) Macomber. S. H.; Furtak, T. E.; Devine, T. M. Chem. Phys. Lett. 1982, 90, 439. Chen, T. T.; Von Raben, K. U.; Owen, J. F.; Chang, R. K.; Laube, B. L. Chem. Phys. Lett. 1982, 9 1 , 494. Cotton, T. M.; Vavra, M., unpublished results. Vo-Dinh, T.; Hiromoto, M. Y. K.; Begun, G. M.; Moody, R. L. Anal. Chem. 1984, 56, 1667. Enlow, P. D.; Buncick, M.; Warmack, R. J.; Vo-Dinh, T. Anal. Chem. 1986. 58. 1119. (15) Boo, D. W.; Oh, W. S.; Kim, M. S., Kim, K. Chem. Phys. Lett. 1985, 120,301. (16) Mahoney, M.; Howard, M. W.; Cooney, R. P. Chem. Phys. Lett. 1980, 71, 59. (17) Pemberton, J. E.; Buck, R . P. Anal. Chem. 1981, 53, 2263. (18) HiMebrandt, P.; Stockburger, M. J . Phys. Chem. 1984, 68,5935. (19) Soriaga, M. P.; Hubbard, A. T. J . Am. Chem. SOC. 1982, 104, 2742. (20) Moskovits, M.; Suh, J. S. J . Phys. Chem. 1984, 86,5526 (21) Cotton, T. M.; Vavra, M. Chem. Phys. Lett. 1984, 106, 491. (22) Cotton, T. M.; Schultz, S.G.; Van Duyne, R. P. J . Am. Chem. SOC. 1982, 104, 6528.
RECEIVED for review August 1,1986. Accepted September 3, 1986. This work was supported by the National Institutes of Health, General Medical Sciences, Grant GM35108, and the National Science Foundation, Grant DMB-8509594. T.M.C. is a recipient of a National Institute of Environmental Health Sciences Research Career Development Award.
Development of a Microdroplet Mixing Technique for the Study of Rapid Reactions by Raman Spectroscopy Stanley F. Simpson, J a m e s R. Kincaid,*' a n d F. J a m e s Holler*
Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506
A new mixing technique for use In the study of fast reactions In solution is described. The technique Is based upon the collision and subsequent coalescence of two highly reproducible streams of mlcrodropiets. The resuning coalesced mlcrodroplet stream passes through an argon ion laser beam, and the progress of a reaction is determined by an analysis of the Raman-scattered ilght. The 10-90% mixlng time has been determlned to be 270 ps for droplet streams collided at 60' by monltorlng an acid-base reaction known to have a diff usion-limited rate constant. The formation curve of the resonance Raman active trls( 1,lO-phenanthroi1ne)-Iron( I I ) complex is presented to demonstrate the potential of the technique for use in kinetic studies.
In a previous report ( I ) , we communicated a preliminary description of a novel mixing technique based on microdroplet generation. The device can be conveniently coupled with Raman spectrometric detection to provide an impressive capability for acquisition of detailed structural information from vibrational spectra of short-lived intermediates. In this experiment, schematically diagrammed in Figure 1,two highPresent address: Department of Chemistry, Marquette University, Milwaukee, WI 53233.
velocity streams of regularly spaced, uniformly sized droplets are oriented such that individual droplets from one stream (reactant A) collide and coalesce with droplets from the second stream (reactant B) to produce a product stream of uniformly sized droplets. The product stream is directed through a focused laser beam, and the scattered light is collected and analyzed with a conventional Raman spectrometer. The entire mixing apparatus can be moved relative to the focused laser beam so as to probe the stream at precise distances and thus precise times after mixing. The performance of the first-generation device originally described was sufficiently impressive (10-90% mixing time ca. 200 p s and time resolution ca. 10 p s ) to encourage us to pursue development. However, the original apparatus lacked an efficient mechanism for fine adjustment of the streams which resulted in laborious acquisition of stream coalescence and poor long-term stream stability. In addition, the methods used to measure droplet spacing, and thus droplet velocity, were unsophisticated. The first method, direct microscopic observation of stroboscopically illuminated droplets, is inherently imprecise ( 2 5 % relative standard deviation), while the second method utilized photographs and therefore provided the droplet velocity only after the film was developed and measurements of the droplet spacing were made on the negative. The purpose of the present report is to present, for the first time, a detailed description of the apparatus employed for the
0003-2700/86/0358-3163$01.50/00 1986 American Chemical Society
