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 C I T E D 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, 9 9 , 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, 5 7 , 783.
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(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, 9 1 , 291. (10) Macomber. S. H.; Furtak, T. E.; Devine, T. M. Chem. Phys. Lett. 1982, 9 0 , 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 Cotton, T. M.; Vavra, M., unpublished results.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. 5 8 . 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, 7 1 , 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 a t 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
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k I
Figure 3. Block diagram of the air and solution flow systems. P.T. represents pressure transducer.
Figure 1. System schematic of the microdroplet experiment. VERTICAL TRANSLATOR PIEZOELECTRIC TRANSDUCER
/
GLASS CAPILLARY HORIZONTAL TRANSLATOR
Flgwe 2. Apparatus for droplet generation and collision. The horiiontal linear translator adjusts the relative phase of the droplet streams while the vertical translator is used to collide the streams.
generation and coalescence of microdroplet streams. The current version incorporates several features which enable rapid and precise measurement of droplet velocities and greatly enhance the ease of acquisition and stability of the coalesced stream. In addition, we report carefully determined characteristic mixing times utilizing a diffusion-controlled acid-base reaction. Finally, the potential utility of the technique for monitoring rapid reactions in solution is demonstrated by the acquisition of the formation curve of the resonance Raman-enhanced tris( 1,lO-phenanthro1ine)-iron(I1) species (2). EXPERIMENTAL SECTION Droplet Generation Apparatus. The most effective method for the generation of uniform droplets utilizes the principle of instability of liquid jets, first described mathematically by Rayleigh (3). Several research groups have utilized these ideas extensively in a variety of applications (4-1 7 ) . In OUT apparatus, the liquid jet is formed by forcing the reagent solution through a specially fabricated glass capillary with compressed gas. The barrel of the capillary is cemented (Duco, Du pont) through a hole drilled in a rectangular piezoelectric strip (Vernitron, Bedford, OH) as shown in Figure 2. The strip is clamped in a cantilever position to a small delrin mount where connections are made to a variable-frequency oscillator. The oscillator is of in-house design and based upon the circuit of Seymour and Boss (17). The application of a square wave of sufficient amplitude and frequency to the bimorph results in a regular axial perturbation along the jet stream, thus establishing the necessary conditions for uniform droplet formation ( 3 ) . The capillary is connected to a 22-gauge stainless-steel syringe needle (Glenco, Houston, TX) using Teflon heat-shrinkable tubing. The needle is housed in a rectangular delrin block and provides the interface between the flow system and the capillary.
In order to successfully collide droplets, three conditions must be satisfied: (1) the streams must be made to intersect; (2) the droplets of each stream must be equally spaced; and (3) they must be in phase. The first and third requirements are satisfied by mounting each generator assembly on a precision linear translator (1-pm resolution) as shown in Figure 2. One translator-generator assembly provides vertical movement and is used to cause the two streams to intersect. The second assembly provides axial movement and allows relative phase adjustment of the two streams. The piezoelectric strips are driven by the same oscillator signal to ensure equal droplet spacing in each stream. Both of the generator-translator mount assemblies are aligned a t the desired collision angle and fastened to an aluminum platform. The platform can be quickly mounted on a three-dimensional stage (Ardel Kinematic Models T-50 translators and PA-32 angle plate), which rests on the optical rail of the Raman spectrometer. In this way, the droplet generators and mounts can be prepositioned and the entire apparatus moved without disturbing the stream alignment. Flow System. A block diagram of the flow system is shown in Figure 3. An empty tank was used as a ballast to minimize pressure fluctuations and improve the precision of the flow rates. A system of valves was constructed so that each reagent solution can be driven by different gases. A high-precision pressure transducer (Omega,Model PX-300-150 G, Stamford, CT) was used to improve the reproducibility of the driving pressure. Two 1.0-pm membrane filters (Millipore, Bedford, MA) housed in stainlesssteel filter holders were used to prevent particulates from clogging the capillaries. Teflon tubing (3.1 mm 0.d. x 1.5 mm i.d.) (Alltech, Deerfield, IL) was used for solution transport, while 1/4-in.-o.d. polyethylene tubing was used to connect the pressure vessel and ballast to the nitrogen supply. Raman Spectroscopic Detection System. The droplet stream was probed by a Spectra Physics Model 165 argon ion laser operating a t 514.5 nm (1.0 W a t the laser). Raman-scattered radiation was collected by a conventional two-lens arrangement and focused on the 500-pm entrance slit of a Spex 1403 double monochromator equipped with a Spex CD2 Compudrive system. Light was detected by an RCA C31034 photomultiplier, and the signal was processed by an ORTEC (Model 9315,9320) photon counting system. An AIM-65 (Rockwell International, Anaheim, CA) microcomputer equipped with BASIC was interfaced to both the photon counter and the CD2 Compudrive unit. Programs were written in BASIC for data collection and subsequent data analysis.
METHODS Reagent Preparation. Sodium hydroxide and sulfuric acid (Baker Reagent Grade) solutions of ca. 1.1 M and 0.5 M, respectively, were prepared and standardized against potassium hydrogen phthalate (Fisher Primary Standard) in the usual manner. Solutions of Fe2+ (ferrous ammonium sulfate, Fisher) and 1,lO-phenanthroline (1,lO-phenanthroline monohydrate, Fisher) were each freshly prepared in 0.25 M Na2S04. The pH of both solutions was checked before each experiment and found to be between 6.5 and 7.0. The Fez+ and phenanthroline solution concentrations were determined spectrophotometrically as Fe(phen)32+(A = 510 nm, t = 11100 M-' cm-') (18) after prefiltration through a 1.0-pm filter. Water obtained from a Milli-Q water purification system (Millipore, Bedford, MA) was used in the preparation of all solutions.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 14, DECEMBER 1986
Droplet Coalescence. Prior to any attempts to attain coalescence, the quality and velocity of the streams were carefully evaluated under a microscope. A strobe light, which was triggered by the oscillator, illuminated each droplet and thus facilitated the measurement of both the size of and spacing between the droplets with a filar eyepiece attached to the microscope. This permits the direct measurement of the volume ratios of the reactant droplets. The velocity of the droplet streams was calculated from the measurement of the spacing and frequency of droplet production measured with a precision frequency meter. The generator assemblies were aligned on the platform at the desired angle and positioned near the coalescence point to minimize the effects of air currents. The streams were made to intersect by careful adjustment of the vertical linear translator (Figure 2). Once obtained, the coalesced stream required only occasional minor adjustments during the experimental session to compensate for pressure and electronic fluctuations. Measurement Technique. Before each experiment, the system was flushed thoroughly with the prefiltered standardized solutions. Because of the corrosive nature of the reagents, the droplet streams were first coalesced at a site away from expensive optical equipment using the procedure previously described. After a coalesced stream of acceptable quality was obtained, the flow was stopped and the aluminum platform was mounted to the stage assembly on the optical rail. The flow was resumed and realignment was performed if necessary to provide a stable stream. The optical system was optimized while observing the 982-cm-' band of the internal standard to yield maximum Raman signal output. The coalescence point was used in all experiments as a reference position representing zero reaction time. Data were collected at precise distances along the coalesced stream. The distance from the point of coalescence was read from a micrometer, which is an integral part of the 3d translation stage. The Raman signal was counted for 10 s at each preselected frequency, and three replicates were performed and averaged to obtain each data point. All experiments were conducted at room temperature, which was 22 f 2 "C. Maximum temperature changes calculated from evaporative mass loss of the droplet stream over a distance of 5 cm corresponding to a time of approximately 5 ms are less than 1.5 "C. Each measurement required approximately 20 mL of solution. R E S U L T S A N D DISCUSSION S t r e a m Stability. The selection of the driving frequency of the droplet generator is critical to the production of uniform droplets. This frequency is dependent upon such parameters as the velocity and viscosity of the solution and the general oscillatory characteristics of the piezoelectric material. Thus, for a given set of experimental conditions, only a small number of oscillator frequencies will yield uniform droplets. Improper selection of the frequency results in nonuniform droplet production or in the generation of satellite droplets as discussed by Lindblad and Schneider (7). In practice, we have found it best t o coalesce droplets a t the highest frequency which results in reproducible droplet formation. The selection of the distances between the capillary tips of the droplet generators and the point of coalescence is also important. We have observed that newly formed droplets oscillate for short distances after detachment from the perturbed liquid jet stream. For this reason, the droplet generators are positioned far enough away from the coalescence point so that these oscillations have sufficiently subsided, yet close enough to minimize the effects of air currents. A distance of ca. 2 cm between the coalescence point and the capillary tip ensures uniform droplet formation before impact while minimizing problems with drafts. In addition, both distances are kept approximately equal to keep any drag effects constant for each stream. Small-bore capillaries periodically clog despite prefiltration and in-line filtration of the reagents. The capillary orifice may be cleaned by inserting wire of ca. 50 pm to dislodge the foreign matter. Subsequent backflushing with filtered distilled water using a 5-mL disposable syringe reopens the flow channel and
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removes the particulates from the system. S t r e a m Coalescence. The process of coalescence is pictured in Figure 1 of ref 1. The droplet streams meet a t the coalescence point in a slightly skewed fashion. At impact, the droplets begin to join as a result of surface tension. At this point, the partially coalesced droplet is "dumbbell" shaped and begins to tumble as the offset collision orientation converts the kinetic energy of the streams to rotational energy. The entire process from impact to coalesced droplet occurs within 150 ps. Drafts and the very fine mist that result from waste droplets spattering against the walls of the droplet-catching mechanism have been found to hinder droplet coalescence. In order to minimize this problem, we have used a 1-L flask with the bottom removed to catch the coalesced stream. A plastic bag with a small hole in the bottom is attached to the bottom of the flask to direct the mixed solution to a waste container. The coalesced droplets enter the mouth of the flask, spatter against the bag, and drip into a waste beaker. In this way, the amount of mist that escapes back in,o the collision area is minimized. Spectral Quality. Initial experiments were undertaken using single microdroplet streams in order to determine the quality of the spectra that might be obtained from an ideally coalesced stream of the microdroplets. The results presented in ref 1 provide a qualitative comparison between Raman spectra obtained from a microdroplet stream and spinning-cell apparatus. Signal-to-noise ratios ( S I N ) were determined by using a 10-s count of the 982-cm-l SO-: band from a solution containing 0.5 mM Fe(phen)32fand 0.25 M S042-.A S I N of 37 was found for the spinning cell, while a value of 22 was calculated for the microdroplet stream. This decrease in SIN is not unreasonable since light is only scattered during the time that a droplet resides in the laser beam. Mixing Time Determination. The mixing time of the microdroplet technique was determined by monitoring the reaction between sulfuric acid and sodium hydroxide, which is known to have a diffusion-limited rate constant of 1 x 10" M-' s-l (19). In this experiment, a microdroplet stream of 1.1 M NaOH was coalesced with a stream of 0.5 M H2S04,and the resulting mixed stream was analyzed. The ratio of the net Raman signal of the HS0,- band at 1053 cm-' to the net Raman signal of the 982-cm-' S042-band was plotted as a function of time to yield the mixing curve. Figure 4 shows three typical mixing curves obtained under the following experimental conditions: 28 psi driving pressures, 60.1" angle of intersection between streams, 140-pm droplet diameter, and coalesced stream velocity of 12.0 m/s. The curves were found to exhibit exponential behavior, and mixing parameters (rmix= 122 9 ws, 10-90% mixing = 268 f 20 ps, 99% mixing = 561 f 42 ps) were obtained by using least-squares data analysis with a logarithmic linearization procedure. As the data show, the reaction reaches 99% completion in ca. 560 ps after the initial contact of the two solutions. Because the coalesced stream can be probed by the laser a t any point after coalescence, and thus there is essentially no dead time, 560 ps represents the mixing time. Optimization of such experimental parameters as droplet velocity, droplet size, and collision angle should make it possible to further shorten the mixing time. It is important to determine the reproducibility of the coalescence process. The mixing curve in Figure 4 represents data taken from three experimental sessions. Between each trial, reagent streams were halted, were deliberately, misaligned, and finally were readjusted. As Figure 4 clearly shows, alignment of the streams between experiments can be accomplished with good precision. The data are least precise during the early portion of the mixing.
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T 0%
rt tI
P
u
'0
05
IO
15
TIME , s x
2.0
25
30
103
Figure 5. Experimental formatiin curve for Fe(phen),"+. Points show the formation of Fe(phen),*+ during the initial 2.5 rns of the reaction M Fe2+ and 8.9 X M 1,lO-phenanthroline. between 1.5 X
the results of the mixing experiments.
CONCLUSION
TIME. S X l o J Figure 4. Mixing curves obtained from observation of the diffusionlimited reaction between HS04- and NaOH. The progress of the mixing
was determined by plotting the ratio of the net intensity of the 1053O : band as cm-' HS04- band to the net intensity of the 982-cm-' S a function of time. Data were obtained from three experimental sessions.
Microdroplet Kinetic Studies. In order to demonstrate the potential of the microdroplet mixing technique for monitoring the reactants and/or the products of rapid reactions in solution, the formation curve of the resonance Raman enhanced Fe(phen)32+complex was recorded. Microdroplet streams of Fez+and 1,lO-phenanthroline, each of which contained 0.25 M SO:-, were coalesced and the resonance Raman spectrum of the resultant droplet stream was analyzed using the sulfate present as an internal standard. Data were treated by calculating the ratio Iphen/Zm4 where Iphn is the net intensity (peak signal minus background) of the 1460-cm-l band of Fe(phen),2+ and Zso, is the net signal of the 982-cm-' SO4'internal standard. Raman signals due to Fe(phen)32+were corrected for reabsorption effects using a calibration curve prepared from a series of Fe(phen)?+ solutions containing 0.25 M SO,2-. A typical Fe(phen)z+ formation curve obtained by plotting as a function of time is presented in Figure corrected Zphen/IS04 5 . The data shown in the figure were taken from two experimental sessions and demonstrate the reproducibility of the technique. The precision of the individual data points ranged from 1 to 20% relative standard deviation and is limited by instability in the coalesced stream during the time required to make a single measurement (-3 min). Such measurements result in relatively low precision rate constants; however, the use of a multichannel detection system should improve the quality of these measurements. The induction period evident during the initial 560 ~s of the reaction curve is due to incomplete mixing of reagents and is consistent with
A new microdroplet mixing technique has been described which shows considerable promise for use in the study of fast reactions in solution. The 99% mixing time for a droplet collision angle of 60" has been found to be 560 ps, which is a factor of 2-5 faster than the mixing time of conventional stopped-flow instruments. Although it is possible to measure rate constants with this technique, the imprecision associated with long data acquisition times required with the apparatus in its present form limits its utility. Improvements in the experimental apparatus including multichannel detection, higher laser powers, and sampling valves are expected to enhance its usefulness. The absence of dead time should permit lower mixing times to be achieved by varying such parameters as the droplet velocity, size, and collision angle. A detailed investigation of the effects of these parameters is in progress in this laboratory.
LITERATURE CITED Simpson, s. F.: Kincaid, J. R.; Holler, F. J. Anal. Chem. 1983, 5 5 , 1420. Clark. R. J. H.; Turtle, P. C.; Strornmen, D. P.; Streusand, B.; Kincaid, J. R.; Nakamoto, K. Inorg. Chem. 1977, 16, 84. Rayleigh, L. Proc. London Math. SOC.1879, 10, 4. Wolf, W. R. Rev. Sci. Instrum. 1961, 32, 1124. Mason, B. J. Endeavor 1964, 23, 136. Park, R. W.; Crosby, E. J. Chem. Eng. S d . 1965, 20, 39. Lindblad, N. R.; Schneider, J. M. J . Sci. Instrum. 1985, 42,635. Lindblad, N. R.; Schneider, J. M. Rev. Sci. Instrum. 1967, 38, 325. Adam, J. R.; Lindbaid, N. R.: Hendricks, C. D. J . Appl. Phys. 1968, 39, 5173. Hieftje, G. M.; Malrnstadt, H. V. Anal. Chem. 1968, 4 0 , 1860. Erin, T.: Hendricks, C. D. Rev. Sci. Instrum. 1988, 39, 1269. Arrowsrnith, A.; Foster, P. J. Chem. Eng. J . (Lusanne) 1973, 5 , 243. Anestos, T. C.; Hendricks, C. D. J . Appl. Phys. 1974, 4 5 , 1176. tiendricks, C. D.;Calliger, R. J.; Robinson, K. S.Bull. Am. Phys. Soc. 1976, 21, 1002. Sangiovanni, 59 J. J.; Kesten, A . S. Combust. Sci. Technol. 1977, 16, Ching, B.: Golay, M. W.; Johnson, T. J. Science (Washington, D . C . ) 1984, 226, 535. Seymour, R. J.; Boss. C. E. Appl. Spectrosc. 1983, 37,375. Brandt, W. W.; Gullstrorn, D. K. J . Am. Chem. SOC.1952, 74,3532. Caldin, E. F. Fast Reactions in Solution: Wiley: New York, 1964.
RECEIVED for review April 8, 1985. Resubmitted September 2, 1986. Accepted September 2, 1986. This work was supported in part by a grant, from the National Science Foundation.
