Matrix-Isolated Surface-Enhanced Raman Spectroscopy (SERS): The Role of the Supporting Matrix
S T E V E N A. S O P E R and T H E O D O R E K U W A N A * The Center for Bioanalytical Chemistry, University of Kansas, Lawrence, Kansas 66046
The influence of a supporting matrix in surface-enhanced Raman spec- troscopy (SERS) has been investigated. The support matrices were con- ventional TLC plates onto which Ag colloidal hydrosols mixed with the dye pararosaniline had been deposited. The protocol of preparation of the Ag sol as well as the type of TLC plate had a profound effect upon the intensity of the SERS signals of pararosaniline. The Ag sol and the TLC plate that resulted in the maximum SERS intensities yielded a detection limit of ~ 108 femtomols (33 pg) of dye deposited onto the TLC plate. Deposition of the dye/sol mixture onto the supporting matrix also resulted in stable SERS signals for extended periods of time, in contrast to the solution-phase case, where the signal is only transient in nature. In order to obtain the SERS spectra, a remote sensing Raman spec- trometer was constructed and is described.
Index Headings: Surface-enhanced Raman spectroscopy; Matrix-iso- lation; TLC; Ag colloids; Raman remote sensing spectrometer.
INTRODUCTION
The utility of Raman spectroscopy, with its rich vi- brational spectral information, has flourished since the discovery of the surfaced-enhanced Raman effect in 1974.1 This effect produces nearly a 6-order enhancement in the Raman scattering intensityY Two general theoretical models have been proposed to account for the observa- tion of the large enhancement. The first model attributes the enhancement to chemical effects brought about by the chemisorption of the scatterer to a metal substrate. 3~ The enhancement is a result of a change in the molecular polarizability of the scatterer, which gives an increased Raman cross section. This model has been labeled the chemical enhancement theory. The second model deals with the enhanced electric field near small dielectric spheres and has been termed the electromagnetic en- hancement model) -14 An increase in the electric field, which yields an increase in the Raman scattering inten- sity, is a direct result of the coupling of the incident electric field to the surface plasmons of the metal. This can only occur efficiently for certain metals (dependent upon their optical properties) with a particular mor- phology.
The SERS effect has been observed for a variety of metal substrates including Ag, Cu, and Au, with Ag being the most commonly used metal. The types of Ag sub- strates that display the enhanced Raman signal include Ag electrodes roughened by an oxidation/reduction cycle in the presence of KC1, chemically prepared Ag films, vapor-deposited Ag films, and colloidal hydrosols. Col- ioidal hydrosols were first shown by Creighton e t al . 1~ to enhance the Raman scattering intensity of pyridine. The
Received 6 March 1989. * Author to whom correspondence should be sent.
advantages of using these colloids are the ease with which they can be prepared and the fact that they can be char- acterized by monitoring their absorption spectrum. The disadvantages of the colloid system are the facts that the enhancement is dependent upon the protocol of prepa- ration and that adsorption of the scatterer onto the col- loidal particle induces aggregation in the colloidal sus- pension. Such aggregation results in a decreased SERS signal as a function of time.
Several different analytical approaches using Ag col- loidal hydrosols for SERS have been reported. Tortes and Winefordner 16 reported on the trace determination of nitrogen-containing drugs with SERS. Good spectra were obtained for uracil and uridine at concentrations as low as 500 ug/mL. Freeman e t al . 17 and Winefordner and co-workers 18,19 used SERS with hydrosols in a flow injection system and reported a limit of detection of 2 ng for the dye pararosaniline. Tran 2°,21 deposited a mix- ture of Ag sol and several different dyes onto filter papers and observed the surface-enhanced Raman spectrum. He found that the SERS signal was dependent upon the method of preparing the Ag colloidal hydrosol, the mode of sampling, and the type of the filter paper. The detec- tion limit was 0.5 ng for the dye crystal violet (e632. 8 =
10,333 M -1 cm -1) deposited onto the filter paper. Along this same line, Laserna e t a l . 22 obtained SERS spectra for p-aminobenzoic acid, acridine, 9-aminoacridine, 2-aminoanthracene, and 5-aminoquinoline on filter pa- per that had previously been coated with Ag sols. The SERS signal of 9-aminoacridine decreased substantially with time, and after ~40 rain, the SERS signal was no longer observable. S6quaris and Koglin 23 obtained SERS spectra of nucleic purine derivatives that had been de- posited onto high-performance thin-layer chromato- graphic plates (HPTLC). The Ag sol was post-deposited onto the plates in order to obtain spectra of the purine derivatives. The detection limits for this system were on the order of 30 ng of material deposited onto the TLC plate.
The purpose of the present communication is to ex- amine the role of the support material, such as various types of thin-layer chromatographic plates (TLC plates), on the adsorbate/Ag hydrosol system. The influence of the matrix on the intensity and the stability of the SERS signal when the adsorbate/sol mixture was deposited onto the TLC plate was of particular interest. Thus, different types of TLC plates and sol preparation protocols were investigated and examined for their influence on the SERS signal. Also, the ability of the TLC plate to pre- serve the SERS signal for extended periods of time wa~ of particular interest. In order that SERS spectra coulc
Fro. 1. Block diagram of the remote-sensing Raman spectrometer. A detailed diagram of the TLC positioner and the collection optics is shown in Fig. 2. FO1 = excitation fiber optic. FO2 = collection optical fibers.
be obtained from the TLC plates, a remote-sensing Ra- man spectrometer was constructed and is described in the present communication.
EXPERIMENTAL
A block diagram of the Raman optical system is shown in Fig. 1. A Spectra-Physics argon-ion laser (Mountain View, CA, Model 171) powered by a Spectra-Physics Model 270 exciter provided the source of excitation. The 514.5-nm emission line of the argon-ion laser was used and was isolated from the plasma lines of the laser with a narrow bandpass filter (FWHM = 10 nm, CWL = 514.5 nm; Omega Optical, Brattleboro, VT). Mounted on the laser head was an X-Y-Z positioner fitted with a micro- scope objective lens for focusing the radiation from the laser onto the fiber optic. Optical fibers were used to carry the excitation light to the TLC plate and to collect the Raman-scattered radiation and carry it to the spec- trometer for analysis. A 400-#m fused-silica optical fiber (General Fiber Optics, Cedar Grove, NJ) was mounted onto the X-Y-Z positioner and transferred the excitation light to the TLC plate. The terminus of this optical fiber was placed into a focusing beam probe (Oriel Corpora- tion, Stratford, CT). The probe was used in order to obtain good spatial resolution for sampling on the TLC plate. This probe consisted of a small metal cylinder containing two matched//1.0 lenses, resulting in a mag- nification of 1.0 and a focused beam spot of ~400 #m on the surface of the TLC plate. Three fused-silica optical fibers (i.d. = 1.0 ram, General Fiber Optics) were posi- tioned in a triangular arrangement and placed in a metal collar for positioning above the TLC plate. The holder for the excitation and collection optical fibers is shown in Fig. 2B (TLC positioner). The focusing beam probe was positioned so that the angle of incidence on the TLC plate was ~45 °. The focused laser spot was situated di- rectly underneath the collection fibers that were posi- tioned 90 ° relative to the plane of the TLC plate. The collection fibers were held ~ 3 mm above the TLC plate by screwing the metal collar onto the holder. The opposite ends of the collection fibers were placed in an
/1.0
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X-Y-Z positioner and aligned in a linear fashion parallel to the slits of the spectrometer.
A diagram of the collection optics is shown in Fig. 2A. The position of the collection fibers was situated at the focal point of L1 (f/1.0 fused-silica lens, Oriel Corp.) by the X-Y-Z positioner. The purpose of L1 was to collimate the radiation from the collection fibers before the scat- tered radiation was sent through the Raman notch filter (the notch filter requires well-collimated light for its ef- fective use). The notch filter was used for rejection of the 514.5-nm scattered radiation (Omega Optical). An additional Corning long-pass colored glass filter was placed after the notch filter to further reject the 514.5- nm scattered radiation (Esco Products, Inc., Oak Ridge, NJ). These two filters were used in conjunction with a double-grating monochromator in order to minimize the high 514.5-nm background resulting from the highly re- flective TLC plates. The scattered radiation was then focused onto the entrance slits of a double-grating mono- chromator (Model DH-10, Instruments SA, Inc., Edison, NJ) with a silica planoconvex lens (Oriel)/-matched to the monochromator, which was driven by a Model 1020- MS microprocessor (Instruments SA). The slits on the double-grating monochromator were fixed at 100 #m and resulted in a spectral resolution of 20 cm -1. The PMT used in the present system was a noncooled Hamamatsu R1527 (Shimokanyo, Japan). The signals were analyzed with the use of photon counting with a Model 1121A amplifier/discriminator and a Model 1109 photon count- er (EG&G Princeton Applied Research, Princeton, NJ). Unless otherwise stated, a 1-s integration time was used, resulting in a scan rate of 3.5 cm-Vs. Data acquisition and monochromator control were performed by a Zenith Z-100 computer. All software was written in-house with Turbo Pascal Version III. Spectral data were subjected to a 9-point Savitzky-Golay 24 digital smoothing proce- dure.
Reagents and Sol Preparation. All TLC plates were obtained from Alltech (Deerfield, IL). AgNO3 used to prepare the sols was gold label and was purchased from Aldrich (Milwaukee, WI), as was pararosaniline (base form). Sodium borohydride, sodium citrate, and potas-
APPLIED SPECTROSCOPY 1181
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R a m a n sh i f t ( c m * * - I FIa. 3. Absorption spectra of t:he Ag colloidal hydrosols used in this investigation. (A) a is NaBH4 sol; b -d are citrate sols for the AgNOa concentration of 0.18 g/L (b), 0.09 g/L (c), and 0.05 g/L (d). (B) The SERS spectra of I x 10 -6 M pararosaniline/sol/KC1 mixture deposited onto a reversed-phase TLC plate (20 ~L) with 35 mW of power on the sample. (a) Citrate sol, 0.18 g/L; (b) NaBH~ sol; (c) citrate sol, 0.05 g/L; and (d) citrate sol, 0.09 g/L.
sium chloride were obtained from Fisher (Fairlawn, NJ). All solutions were made in NANOpure water (Sybron Systems, Boston, MA).
The sodium borohydride sols were prepared according to the published procedure of Creighton et al.15 The ci- trate sols were prepared by the method suggested by Lee and Meisel, 25 which is briefly outlined here. The appro- priate amount of AgNO3 was added to 500 mL of boiling water, which was stirred vigorously while being purged with N~. A 1% solution of sodium citrate was added dropwise to the AgNO3 solution at a drip rate of ~1 drop/15 s. After the sodium citrate addition was com- pleted, the solution was boiled for an additional 45 min with rapid stirring. The sol was then cooled to room temperature and stored in a capped bottle. The sols pre- pared by this procedure had an absorption maxima at ~410 nm and could be sl;ored for several weeks.
The general protocol for obtaining the SERS spectra involved the addition of KC1 to 3.0 mL of the sol (the amount of KCI added was equivalent to the amount of Ag contained within the 3-mL portion). After the acti-
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vation of the sol by KC1, the appropriate amount of the dye (pararosaniline) was added to the sol. Upon the ad- dition of the dye, the sol immediately began to change color from a dark yellow/green to gray, indicating aggre- gation of the sol. In all cases, no aggregation of the sol after the addition of the dye was observed for sols that had not been activated with KC1. The dye/sol/KC1 sys- tem was then deposited onto the appropriate TLC plate, followed by acquisition of the SERS spectrum.
RESULTS AND DISCUSSION
Pararosaniline was selected as the model compound for this study. It has a molar absorptivity of ~ 1000 cm-' M -1 at the excitation wavelength of 514.5 nm with neg- ligible fluorescence emission. Initially, we investigated the method of sol preparation aimed at maximizing the SERS intensity. Ag sols investigated were prepared with sodium borohydride or citrate with various AgN03 con- centrations. The electronic spectra of the four different sol preparations are shown in Fig. 3A. The sodium bo- rohydride sol resulted in a very narrow absorption band (FWHM = 50 nm) with a maximum at ~ 390 nm, indic- ative of monodispersed particles with a diameter of ~ 20 nm. 2~,2G In Fig. 3A, b-d, the absorption spectra for citrate sols at Ag concentrations of 0.05 g/L, 0.09 g/L and 0.18 g/L are shown. The absorption maxima are at ~410 nm for all three cases, with the absorption at 410 nm in- creasing with increasing Ag concentrations. SEM micro- graphs indicated a particle diameter of ~ 210 nm for the citrate sols. For the case of the Ag sol with the highest Ag concentration, one observed a large degree of aggre- gation of the Ag particles, indicated by the very broad absorption band. In Fig. 3B, the SERS spectrum of the pararosaniline/sol/KC1 mixture deposited onto a re- versed-phase TLC plate (20 #L) is seen. No evidence of Raman bands from pararosaniline was observed for the sodium borohydride or the citrate sol when the Ag con- centration was 0.05 g/L. The bands at 1525 and 1593 cm -1 are only slightly visible for the citrate sol with a Ag concentration of 0.09 g/L. The most intense Raman bands were found for the sol prepared with citrate when the Ag concentration was 0.18 g/L. The results suggest that
1182 Volume 43, Number 7, 1989
50000.
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, I , , I , , I , i I 1500. 1600. 1700. 1800. Roman sh i f t ( c m * * - 1 )
FIG. 5. SERS spectra of 1 x 10 -6 M pararosaniline deposited (20 pL) onto various types of TLC plates; (a) aluminum oxide TLC, (b) silica TLC, and (c) reversed-phase TLC (100 mW of power on the sample).
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FIG. 6. Comparison of the matrix-isolated (aluminum oxide TLC) SERS spectrum of pararosaniline (20/~L of i x 10-~ M deposited onto the TLC) with the solution-phase SERS spectrum (35 mW of power on the sample).
the maximum SERS intensity occurs when the sol has a large absorption at the wavelength of the incident ra- diation and a partial degree of sol aggregation, as was evident from the absorption spectrum of the sol. The large absorbance values of the sol at the laser frequency indicate efficient coupling of the incident electric field to the surface plasmons of the colloidal particle, while partial aggregation of the sol has been shown to display the largest degree of enhancement. 2~ Laserna e t a l . 27 have shown that, for borohydride sols, partially aggregated sols resulted in a more intense SERS signal. For the remainder of this work, sols prepared with citrate at a Ag concentration of 0.18 g/L were used.
The solution and matrix-isolated SERS spectra of pararosaniline are shown in Fig. 4. The solution-phase spectrum was acquired with the instrumentation previ- ously described (the TLC plate was replaced by a small piece of acrylic, which had a hole bored through it and glass microscope slips sealed onto it for holding the so- lution sample). A reversed-phase TLC plate served as the isolation matrix onto which 20 pL of the pararosani- line/sol/KC1 mixture had been deposited. The spectrum for the TLC matrix is very similar in terms of the band positions to that of the solution phase case, but the band intensities of the matrix-isolated spectrum at higher wave- shifts show a decreased intensity. The total aquisition time of each spectrum was ~ 10 min. Therefore, the loss in intensity as a function of time (larger wave-shifts) in the matrix-isolated spectrum could result from the pho- todegradation of the Ag/amine complex (exposure to the high photon flux of the laser), the instability of the Ag/ amine complex in this matrix, or a decrease in the di- electric constant of the surrounding medium.
The following equation has been suggested for describ- ing the enhancement factor at resonance for a spher- oid: 11
If,'es 12 = [167r/(2 +co) 2] (Wp/l-)4(a/R) 12 (1)
where t ies = enhancement factor at resonance; eo = the ambient dielectric constant; ~p = surface plasmon fre- quency; r -1 = the plasmon damping rate; a = sphere radius; and R = distance from the molecule to the center
of the spheroid. The enhancement factor is directly de- pendent on the surrounding dielectric constant. In the case of the matrix-isolated SERS spectrum, the aqueous environment of the Ag/amine complex is replaced by air. The decrease in the dielectric constant resulting from the replacement of water by air produces a loss in the SERS signal.
In an at tempt to maximize the SERS signal, various types of TLC plates were investigated. The SERS spectra of pararosaniline on three different types of TLC plates are shown in Fig. 5. As can be seen, the aluminum oxide plate gave a much larger SERS signal, compared to the other two plates. The background-corrected intensity for the 1593-cm -1 band was 18,800 cps for the aluminum oxide plate, while the silica plate gave a signal of 6000 cps. The reversed-phase plate resulted in a signal of 1500 cps. A comparison of the SERS intensity of the 1593- cm -I band of pararosaniline was also made between an aluminum oxide and HPTLC plate (the HPTLC plate has a smaller particle size, compared to the aluminum oxide plate, and results in a higher percentage of the deposited material exposed to the incident radiation). The aluminum oxide plate gave a higher SERS signal (intensity of the 1593-cm-' band on the HPTLC was 17,000 cps for a 20-~L deposition of 1 x 10 -s M para- rosaniline). The matrix-isolated (aluminum oxide TLC) and the solution-phase SERS spectra for pararosaniline are compared in Fig. 6. The matrix-isolated spectrum is very similar to the solution-phase spectrum in terms of the band positions, but the intensity of the bands at lower wave-shifts are more intense for the matrix-isolated spec- trum than the solution case. The bands at higher wave- shifts again showed a decrease in intensity due to reasons cited above.
The intense signals observed in the case of the alu- minum oxide plate, compared to the silica-based TLC plate, could be attributed to a pH effect. The alkalinity of the aluminum oxide TLC plate would facilitate the adsorption of pararosaniline onto the sol (pH = 9.0), while the silica plate (pH = 4.0) would inhibit the ad- sorption of the dye onto the sol, due to protonation of
APPLIED SPECTROSCOPY 1183
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Time (rain) FIG. 7. In t ens i ty vs. t ime prolile for the background-cor rec ted 1593- cm -1 band of I x 10 6 M pararosani l ine deposi ted (20/~L) onto a re- versed-phase T L C A and an a l u m i n u m oxide T L C B wi th a laser power of 100 m W on the ~ m p l e . For C, the condi t ions are the same as in B, b u t wi th 35 m W of laser power on the sample. In A, B, and C, d i amonds = T L C plate, t r iangles = T L C + ac t iva ted sol ,-and c i rc l~ = T L C + ac t iva ted sol + pararosani l ine.
the amine groups of the dye. These results suggest that pararosaniline preferentially adsorbs onto the Ag par- ticles as the unprotonated form. The marked increase in the SERS signal of the Ag/amine complex deposited onto
the aluminum oxide plate, as compared with the re- versed-phase plate, is probably due to the ambient di- electric constant of the aluminum oxide being much greater than that of the reversed-phase TLC plate. This supposition would be consistent with the hydrophilic na- ture of aluminum oxide and the very hydrophobic nature of the reversed-phase TLC. The increased intensity of the Raman bands observed for the matrix-isolated spec- trum (aluminum oxide plate), compared with the solu- tion-phase spectrum, cannot be attributed solely to a pH effect, since the pH of the sol/KC1 system (pH = 6.80) is near that of the basic aluminum oxide TLC. The de- creased intensity of the solution-phase spectrum could be a result of the reabsorption of the Raman-scattered radiation by a very optically dense Ag sol. The focused laser spot is ~ 1 mm from the front wall of the cell used for acquisition of the solution-phase spectrum, thereby allowing the Ag sol to reabsorb the Raman-scattered ra- diation before it can be collected by the optical fibers. The reabsorption of the Raman-scattered radiation is not observed on the TLC plate since the laser is focused onto the surface of the plate.
The stability of the SERS signal on the reversed-phase and the aluminum oxide TLC plates was then investi- gated. The background-corrected intensity of the 1593- cm -1 band of pararosaniline was monitored as a function of time with constant irradiation (100 mW on the sam- ple). In Fig. 7A the SERS signal of the dye on a reversed- phase TLC plate is shown. The signal decays to baseline within 7.5 min. Figure 7B shows the intensity vs. time profile for the aluminum oxide TLC plate. In this case, one can observe a SERS signal for 30 min after the de- position of the material onto the aluminum oxide plate. These results indicate that the Ag/amine complex is not stable in the reversed-phase matrix. The instability of the Ag/amine complex is most likely a consequence of competitive adsorption onto the Ag colloid between the dye and the hydrocarbon phase of the reversed-phase plate. This competitive adsorption process is not ob- served when the Ag/amine complex is deposited onto the aluminum oxide plate. For the aluminum oxide at ~ 2.0 min, there is a sharp decrease in the SERS signal, due to evaporation of the water medium surrounding the Ag/ amine complex. It is interesting that the SERS signal increases immediately after the deposition of the Ag/ amine complex onto the aluminum oxide plate. This in- dicates that the aluminum oxide matrix facilitates the adsorption of the dye onto the sol, resulting in an en- hanced SERS signal. Figure 7C shows the intensity vs. time profile for the 1593-cm -1 band recorded under the same conditions as in Fig. 7B but with an incident power of 35 mW. As was seen in Fig. 7B, the SERS intensity increased immediately after deposition of the Ag/amine complex, followed by a constant signal and then a sharp decrease in the signal after ~ 5 min. However, the back- ground-corrected intensity of the 1593-cm -~ band of pararosaniline resulting 30 min after deposition was ap- proximately the same for both incident laser powers of 35 and 100 mW on the sample. Comparison of Fig. 7B and 7C indicates that the evaporation process and the rate of photodecomposition (as indicated by the decrease in the SERS signal) was greater for higher laser powers. Comparison of the S/N ratio for the laser powers of 100
1184 Volume 43, Number 7, 1989
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Raman shift ( c m * * - l ) FIO. 8. SERS spect ra of 1 x 10 6 M pararosani l ine deposi ted onto an a l u m i n u m oxide TLC (20 #L) with 100 m W of laser power on the sample. The spect ra were acquired 2 h af ter deposit ion. (a) Wet, ad- di t ion of 20 ~L of H20 to the Ag/amine spot; (b) dry.
and 35 mW on the sample showed that higher laser pow- ers resulted in better S/N ratios. Deposition of 20 gL of 5 × 10 -s M pararosaniline onto an aluminum oxide TLC showed that the S/N ratio for the 1593-cm -~ band was 4.5 at 35 mW and 23.9 at 100 mW incident photon fluxes. Therefore, lower limits of detection may be obtained for higher photon fluxes, although longer exposure of the Ag/amine complex to the higher photon flux degraded the signal at a faster rate.
The degradation of the SERS intensity is probably caused by evaporation of the surrounding water medium, giving rise to a smaller value of the ambient dielectric constant. Therefore, the addition of H20 should restore the SERS signal. A partial restoration of the SERS signal for 9-aminoacridine was observed by Laserna e t al . 22 upon addition of H20 to the system. Figure 8 shows results for the pararosaniline/sol/KC1 system. Pararosaniline (1 x 10 ~ M) was deposited onto the aluminum oxide TLC plate and stored in the dark for 2 h to remove the H20. The SERS spectrum was recorded on the dry deposited spot and after the addition of 20 gL of H20 to the de- posited material. The addition of H20 restored the in- tensity of the SERS spectrum to that obtained imme- diately after deposition of the dye onto the matrix. Repeating this same experiment for the Ag/amine com- plex deposited onto the HPTLC, which yielded SERS intensities for pararosaniline similar to that obtained for aluminum oxide, was unsuccessful. It was discovered that the SERS spectrum could not be observed ~ 20 min after deposition of the material onto the HPTLC matrix and that the addition of H20 resulted only in a slight res- toration (~5%) of the SERS intensity. Therefore, not only did the aluminum oxide TLC plate give the maxi- mum SERS intensity of the TLC plates investigated but also the SERS spectrum of the dye could be observed after 4 days of storage on the plate.
The stablity of the SERS signal for the pararosaniline/ sol/KC1 system was then compared for the solution-phase and the matrix-isolated cases. For the matrix-isolated case, pararosaniline was added to the activated Ag sol, followed by the deposition of 20 gL of the mixture onto the aluminum oxide TLC plate. The deposited material
Time (min) FIG. 9. (A) The absorpt ion spect ra of the pararosani l ine/sol /KC1 sys- t em t aken a t different t ime intervals after the addi t ion of the dye to the ac t iva ted sol; a = 1 rain, b = 10 min , c = 20 min, d = 30 min , e = 60 min, and f = 120 min. (B) The background-correc ted in tens i ty of the 1593-cm -1 band of pararosani l ine a t different t ime intervals after the addi t ion of the dye to the ac t iva ted sol (circles = deposi t ion of the mater ia l onto the T L C immedia te ly af ter the addi t ion of the dye and sampled a t various t imes, squares = deposi t ion of the dye/ac t iva ted sol mix tu re onto the T L C at various t imes followed by acquisi t ion of the spec t rum) . Inc ident pho ton flux = 100 mW, deposi t ion of 20 ~L onto an a l u m i n u m oxide TLC.
was then sampled at various time intervals after the de- position of the pararosaniline/sol/KC1 mixture to the TLC plate (before the acquisition of the spectrum, 20 gL of H20 was added to the deposited material). For the so- lution-phase case, 20 #L of the pararosaniline/sol/KC1 mixture was deposited onto the aluminum oxide TLC plate at various time intervals after the addition of para- rosaniline to the activated sol (spectral acquisition was then performed immediately after deposition). Figure 9A shows the absorption spectra of the pararosaniline/sol/ KC1 system taken at different time intervals. As is evi- dent from the spectra, the value of the absorption at 514.5 nm decreases as a function of time, while the ab- sorption values at longer wavelengths (not shown in Fig. 9A) increase. This results from the continued aggregation of the sol produced by the adsorption of the dye onto the colloidal particles. The background-corrected inten-
s i t y of the 1593-cm -1 band of pararosaniline as a function
Raman shift ( c m * * - l ) Fro. 10. The SERS spectra of pararosaniline (a = 5.0 x 10 -~ M, b = 1.0 x 10 -8 M) for 20 uL depostited onto an aluminum oxide TLC (100 mW on sample).
of time is shown in Fig. 9E;. The solution-phase intensity is observed to decrease dramatically with time, due to the aggregation of the colloidal particles. This aggrega- tion causes inefficiency in the coupling of the incident electric field to the surface EM modes of Ag (small ab- sorption values at the laser frequency).
In the case of the matrix-isolated intensity at 1593 cm -1, one notices that the signal initially increases, and after a 2-h period, the intensity is nearly the same as it is immediately after the addition of the dye to the ac- tivated sol. The reason is that the supporting matrix inhibits the continued aggregation of the sol once it has been deposited onto the matrix, thereby stabilizing the SERS signal.
Various concentrations of pararosaniline were added to the activated sol, and 20 #L was deposited onto the aluminum oxide TLC plate in order to investigate the linearity of the response and to obtain the detection limits for the system. The calibration plot was linear over 3 orders of magnitude in concentration from 10 -5 M to 10 -s M. The SERS spectra of 5 x 10 -s M and 1 x 10 -s M pararosaniline are shown in Fig. 10. With the 1593- cm -1 band of pararosaniline the S/N ratio is 5.6 for a 20- gL deposition of 1 x 10 -s M. The detection limit (S/N = 3) is 5.4 x 10 -9 M of pararosaniline (20 t~L deposition of this concentration re,,~ults in 108 femtomols of dye deposited onto the TLC plate or 33 pg of material). To our knowledge, this is the lowest detection limit reported to date with the use of SERS and Ag colloidal hydrosols as the enhancing substrate. This detection limit can fur- ther be improved by deposition of smaller volumes of material onto the TLC plate and incorporation of a higher- resolution spectrometer. Since the incident radiation is focused to a spot with a diameter of ~400 #m and the deposition of 20 ttL resulted in a spot diameter of ~2 mm, only a small percentage of the deposited material is actually sampled. Therefore, the deposition of smaller volumes of material will result in even lower detection limits. The lens system shown in Fig. 2A gives a magnified image of the radiation emanating from the collection fibers on the entrance slits of the monochromator of ~ 3- 4 mm in diameter. Coupled with the 100-ttm slit width
of the monochromator, this results in a large percentage of the collected radiation not being processed. Therefore, the incorporation of a high-resolution monochromator (wider slit widths) will result in higher throughput of the collected scattered radiation.
CONCLUSIONS
It has been shown that a TLC supporting matrix with colloidal hydrosols as the enhancing substrate offers some important analytical advantages for SERS. The sup- porting matrix has been shown to stabilize the SERS signal for the sol system by inhibiting the continued aggregation of the colloidal particles. In solution such aggregation results in a decreased SERS signal.
With an appropriate choice of a supporting matrix, which was found to be an aluminum oxide TLC plate, enhanced SERS intensities have been observed. The ma- trix plays an important role in the adsorption process of the adsorbate to the Ag substrate, making it possible to analyze small quantities of analytes deposited onto TLC plates. The advantages of the matrix-isolated SERS sys- tem are being utilized in a combination LC/TLC system. The effluent of an LC is deposited onto the TLC plate (after being mixed with the activated sol), which then serves as the isolation matrix. The SERS analysis of the LC analytes deposited provides qualitative (structural information from the vibrational Raman spectrum) and quantitative information. The characteristics and appli- cations of the LC/TLC/SERS system will be reported in a future publication.
ACKNOWLEDGMENTS
The authors wish to express their gratitude to Ken Ratzlaff and Tom Peters for their help in writing the computer software and to Prof. George Strojek for help in the construction of the TLC positioner. The authors would also like to express their gratitude to Prof. Van Duyne and Bob Freeman for their helpful technical discussions during the course of this work. The financial support of Oread Laboratories {Law- rence, KS), the Kansas Technology Enterprise Corporation and the National Science Foundation is greatly appreciated.
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Photodiode Array Systems for Inductively Coupled Plasma-Atomic Emission Spectrometry
K E I T H C. L E P L A and G A R Y H O R L I C K * Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Details are presented for the construction of photodiode array (PDA) measurement systems from commercial components. The PDA systems described include the Hamamatsu $2304-1024Q, a Reticon 1024S using the RC1000 and RC1001 circuit boards, and a Reticon 1024S using the RC1024S circuit board. Detals are presented for computer-controlled clocking and timing circuits, ADC sub-systems, and Peltier cooling sub- systems. The measurement characteristics (sensitivity and detection lim- its) for all arrays are intercompared with the use of analyte emission signals from an inductively coupled plasma. Index Headings: Instrumentation, emission spectroscopy.
INTRODUCTION
Since they were first developed in 1971, linear pho- todiode arrays (PDAs) have been used as detectors in atomic emission spectrometers. 1 The basic PDA spec- trometer is one in which a single PDA is mounted in the exit focal plane of a simple plane grating spectrometer, as shown in Fig. 1. A key development occurred in 1978 when Reticon introduced the 1024S PDA, which has been the dominant PDA device for about a decade. One of the first characterizations of this device was by Talmi and Simpson, 2 and in the early 1980s McGeorge and Salin published a series of papers detailing the performance of this device for atomic spectrochemical measure- ments? -8 It has since been used by numerous researchers and incorporated into a number of commercial products. In particular, PDA spectrometers based on this device have been used in conjunction with the inductively cou- pled plasma (ICP) source. Studies include an overall performance evaluation with the ICP, 9 ICP spectral char- acterization, 1°,1~ transient signal measurements, 12 and near-IR spectral measurements? s-~7 PDA spectrometers have also been particularly useful for several fundamen- tal studies of the ICP including the measurement of ver- tical spatial emission profiles, ~s-2~ lateral spatial emission profiles with subsequent Abel inversion, 22-24 background spectra, 25 signal-to-noise ratios, 26 and H~ linewidths for electron densi ty measurements . 27 Finally, tempera- tures, 2s,29 spatially resolved electron densities, 3° analyte ionization, 31 and excitation conditions in low-flow torches
Received 12 March 1989. * Author to whom correspondence should be sent.
have all been measured with the aid of PDA spectrom- eters. 32
Many of these plasma studies would not have been possible, if not for the multichannel capabilities of the PDA. The diagnostic study of a single parameter may require several thousand individual intensity measure- ments. In the study by Choot and Horlick, ~7 for example, electron density maps were measured at 90 spatial po- sitions in the ICP. The PDA spectrometer was used to acquire a multiwavelength spectrum at each position. Without the PDA, a single electron density map would have required 5760 individual intensity measurements.
Most of the studies noted above were carried out with laboratory-constructed PDA spectrometer systems. In this paper, details are presented on how to assemble a PDA system from commercial components. The focus is on two relatively new PDA systems, the Hamamatsu $2304-1024Q PDA and a Reticon 1024S system using the new RC1000 and RC1001 circuit boards. Details of the procedure for mounting the arrays in the spectrometer focal plane, the Peltier-based cooling sub-systems for the arrays, the clocking systems, and the configuration for interfacing the arrays to an IBM-PC are presented. Then, the operational and measurement characteristics of all arrays for ICP-AES are briefly compared, including re-
Grating Folding mirror
Fro. 1. Schematic diagram of Czerny-Turner type monochromator with a photodiode array (PDA) detector.
