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A Silver Colloid Produced by Reduction with Hydrazine as Support for Highly Sensitive Surface-Enhanced Raman Spectroscopy† Ulrich Nickel,* Amelie zu Castell, Karin Po¨ppl, and Siegfried Schneider Institute of Physical and Theoretical Chemistry, University Erlangen-Nuremberg, D-91058 Erlangen, Germany Received April 10, 2000. In Final Form: July 10, 2000 Reduction of aqueous silver nitrate by hydrazine dihydrochloride in weakly alkaline solution results in a polydisperse colloid that is stable for many months without addition of any stabilizing compounds. The average size of the predominantly spherical particles depends on the initial concentration of silver ions, ranging between 40 and 70 nm in diameter. The colloidal solutions exhibit a characteristic absorption in the blue region of the visible spectrum and are not turbid below a formal silver concentration of 4.5 × 10-4 M. With colloids prepared from 1.5 × 10-4 M silver(I), the SERS spectra of dyes such as nile blue A could be recorded from a solution with concentrations as low as 10-10 M, whereas no SERS signal was observed for dye concentrations higher than 10-4 M. The maximum signal intensity was obtained at a concentration of about 10-7 M. With colloids prepared from g3 × 10-4 M silver(I), no SERS signal was obtained from highly diluted solutions, but the concentration limit for the maximum signal intensity of around 10-7 M became even sharper. The thus prepared silver colloids can therefore be recommended for qualitative detection of certain organic compounds in the parts per billion range as well as for a semiquantitative determination in the parts per million range.
Introduction The characterization of organic compounds by means of surface-enhanced Raman scattering (SERS)1,2 is very attractive because, in contrast to IR spectroscopy, this method can be applied to aqueous solutions. A prerequisite for making use of the SERS effect is the adsorption of the compound under investigation at the properly roughened surface of silver or some other coin metal. Several procedures have been proposed to prepare a suitable SERS-active support.3-7 Very often, silver electrodes or silver colloids have been used because the enhancement factor can be very high for probe lasers working around 500 to 600 nm. In some cases, SERS (or SERRS: surfaceenhanced resonance Raman scattering) signals could be detected for a compound present at a concentration as low as 10-12 M.8-10 However, in routine applications the detection sensitivity most often is not so high. With silver colloids as the support, the analyte concentration usually has to be about 10-7 M to obtain an interpretable Raman signal.11 * Corresponding author. † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. (1) Brandt, E. S.; Cotton, T. M. In Investigations of Surfaces and Interfaces: Part B; Rossiter, B. W., Baetzold, R. C., Eds.; Wiley: New York, 1993; p 633. (2) Campion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241. (3) Sauer, G.; Schneider, S.; Nickel, U. J. Raman Spectrosc. in press. (4) Van Duyne, R. P.; Hultreen, J. C.; Treichel, D. A. J. Chem. Phys. 1993, 99, 2101. (5) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26, 163. (6) Creighton, J. A.; Blatchford, C. E.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790. (7) Reents, B.; Lacconi, G.; Plieth, W. J. Electroanal. Chem. 1994, 376, 185. (8) Kneipp, K. Exp. Technik Phys. 1988, 36, 161. (9) Kneipp, K.; Kneipp, H.; Manoharan, R.; Itzkan, I.; Dasari, R. R.; Feld, M. S. J. Raman Spectrosc. 1998, 29, 743. (10) Hildebrandt, P.; Stockburger, M. J. Phys. Chem. 1984, 88, 5935. (11) Schneider, S.; Halbig, P.; Grau, H.; Nickel, U. Photochem. Photobiol. 1994, 60, 605.
A widely applied preparation technique is the reduction of silver nitrate with citric acid.12 This technique yields a polydisperse colloid that is useful for a qualitative investigation, but its turbidity sometimes disturbes optical measurements and the addition of analyte can easily cause aggregation and sedimentation. As a consequence, performing a quantitative analysis with colloids of this kind is extremely difficult, if possible at all. To overcome these problems, monodisperse colloids with particle sizes ranging from a few to several hundred nanometers in diameter have been prepared,11 but the sensitivity obtained so far is not high enough to compete successfully with other established techniques. In this paper, we present a colloid prepared by reduction of silver nitrate with hydrazine. This method has already been often applied, however, usually with hydrazine hydrate or sulfate and sometimes with the aim of producing very small particles.13-16 To achieve a good SERS enhancement factor, the particle size must not be too small; therefore, we employed the procedure of reducing silver nitrate with hydrazine dihydrochloride, which yields larger particles because the reaction rate can be retarded. This may allow a highly sensitive qualitative analysis of organic compounds by means of SERS spectroscopy as well as the semiquantitative determination in the parts per million range. Experimental Section (a) Materials. All reagents of greater than 99% purity were used without further purification. The solutions were prepared with doubly distilled water and were saturated with argon. All reactions were performed under an argon atmosphere. Silver nitrate and hydrazine dihydrochoride were supplied by Aldrich. (12) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (13) Zhang, Z. T.; Zhao, B.; Hu, L. M. J. Solid State Chem. 1996, 121, 105. (14) Gutbier, A. Kolloid-Z. 1909, 4, 308. (15) Kohlschu¨tter, V. Liebigs Ann. Chem. 1912, 387, 86. (16) Pal, T.; Maity, D. S.; Ganguly, A. Analyst 1986, 111, 1413.
10.1021/la000536y CCC: $19.00 © 2000 American Chemical Society Published on Web 11/07/2000
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Sodium hydroxide (0.1 M), potassium chloride, and potassium iodide were purchased from Merck; potassium bromide was obtained from Fluka. The dye nile blue A, whose structure is given below, was supplied from Aldrich.
(b) Methods. The UV-vis spectra were recorded with a J&M TIDAS spectrometer. The SERS spectra were obtained in a backscattering geometry on a homemade apparatus employing 532.6-nm radiation from a pumped solid-state diode microlaser. After being dispersed by means of a Jobin Yvon HR 320 double monochromator, the scattered light was detected by a diodearray IRY-700 (Spectroscopy Instruments), applying a 1-s integration time. All spectroscopic experiments were carried out at room temperature (about 22 °C) using polyacrylic cuvettes from Sarstedt with an optical path length of 10 mm. The size of the silver particles was determined by means of a Philips EM 400 transmission electron microscope.
Results and Discussion (a) Preparation of the Silver Colloids. To determine the optimum conditions for preparation of both stable and highly SERS-active silver colloids, a large number of experiments were carried out, varying both the concentration of the reagents and the pH. Usually, as the first step, 1 mL aqueous silver nitrate was placed in a polyacrylic cuvette. The reaction was started by the addition of 1 mL of a hydrazine solution, whose pH had been adjusted with some microliters of 0.1 M NaOH. Of course, the final pH in the reaction mixture was lower because of the release of protons during the redox reaction:
4Ag+ + N2H4 w 4Ag0 + N2 + 4H+
(1)
According to this equation, the molar ratio of silver nitrate/hydrazine was always set lower than 4:1 to avoid residual silver(I). The initial pH was varied from 6 to 12. The concentration of silver nitrate in the mixture was chosen between 1.5 × 10-4 and 4.5 × 10-4 M and that of hydrazine between 1.9 × 10-4 and 5.6 × 10-4 M. Most often, no stable colloids were formed and the UV-vis spectra changed continuously. Colloids with a formal silver concentration greater than 10-3 M were turbid. Sometimes, deposition of silver, silver chloride, and/or silver hydroxide occurred. In the pH range between 7 and 10.5 (measured 1 day after mixing the reactants), stable and transparent colloids could be prepared reproducibly. Furthermore, the addition of other components, such as chloride, bromide, and iodide, with concentrations lower than 10-3 M did not noticeably influence the absorption. Figure 1 shows the optical spectra of a silver colloid of this kind recorded immediately after mixing the reagents (dashed line), 30 min later (dotted line), and after 1 day (solid line). Immediately after the mixing process, no plasmon band of the colloid can be observed; only the scattering of intermediately formed AgCl is shown. This contention is proved by optical spectra of a mixture of AgNO3 and KCl in analogous concentrations. This polydisperse colloid with a final pH of 10.2 was found to be stable for several months, that is, no further change of the visible spectrum occurred. The small spectral changes observed during the first day are caused by Ostwald ripening. The long wavelength tail, which is indicative of larger particles or particles
Figure 1. Optical spectra of a mixture of equal volumes of 3 × 10-4 M silver nitrate and 3.75 × 10-4 M hydrazine dihydrochloride/NaOH (pH ) 10.8). The final formal concentration of silver in the colloid is 1.5 × 10-4 M, and the final pH is 10.2.
with high aspect ratios, disappears, and the absorbance around the maximum is increased. From the wellestablished correlation between λmax and particle size, one can estimate the particle diameter of the colloid presented in Figure 1 as somewhere around 40 nm.17,18 This estimate is verified by the TEM micrograph displayed in Figure 2a. This micrograph shows that the particles are predominantly spherical with some variation in size, the average value being about 40 nm. This is significantly different from widely used colloids prepared, for example, according to the procedure of Lee and Meisel12 or Hildebrandt and Stockburger.10 For comparison, the TEM micrograph of a colloid of this kind is displayed in Figure 2b. In this colloid, one finds a great variety of shapes, from spheres and cubes to rods and needles. The shape variation cannot be controlled but is in terms of the electromagnetic theory of SERS an important parameter for the surface enhancement (see below). The amount of silver in this type of colloid is about 10-fold higher than in ours. This implies that UV-vis absorption spectroscopy cannot be used as an easy-to-handle technique for colloid characterization. Furthermore, upon dilution with water, colloids of this kind often become instable, and sedimentation occurs. The formation of silver chloride and silver hydroxide (at a pH > 8) occurs during mixing of the reactants, that is, at the beginning, no redox reaction takes place. Thus, the absorption measured immediately after carrying out the mixing process (dashed line) equals that of a mixture in which potasssium chloride is used instead of hydrazine dihydrochloride. Furthermore, this initial spectrum depends little upon the pH (concurrence of the formation of silver chloride and silver hydroxide) but strongly on the ratio of silver nitrate/hydrazine dihydrochloride. The consecutive redox reaction competes with the growth of the silver chloride and silver hydroxide particles. At low pH, that is, when not all the hydrogen chloride has been neutralized with sodium hydroxide, hydrazine remains partially protonated. Because of the high redox potential of protonated hydrazine, electron transfer to the silver halide, whose redox potential is lower than that of free silver ions, is unfavorable. Therefore, under these conditions the growth of silver chloride dominates. With increasing pH, the actual redox potential of the reducing agent decreases, and hydrazine will be able to reduce silver chloride. However, this reaction is inhibited because of (17) Creighton, J. A. In Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum Press: New York, 1982; p 315. (18) Kerker, M. J. Colloid Interface Sci. 1985, 105, 297.
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Figure 2. (a) TEM micrographs of the colloid, whose spectrum is displayed in Figure 1. (b) TEM micrographs of a colloid prepared according to the procedure proposed by Lee and Meisel12 by reduction of 1 × 10-3 M silver nitrate with sodium citrate.
the release of protons according to eq 1. Therefore, the initial pH should be higher than 10. Under these conditions, it seems possible to reduce all of the silver halide. Of course, the initial formation rate of the colloid increases with increasing concentration of the reactants and so does the average size of the silver particles finally obtained. This indicates that the growing process predominates over the reduction of silver(I), but detailed kinetic studies have not been carried out so far. At a very high pH, the formation of silver hydroxide becomes dominant and suppresses the consecutive redox reaction. Therefore, with an initial pH > 12, no stable colloids could be obtained. To examine the reproducibility of the colloid preparation, usually more than 30 samples were prepared in parallel in the described manner. The absorbance was recorded for each sample. After 1 day, usually only two or three of these samples exhibited more than 5% deviation in the visible spectrum. These samples were discarded, while the other ones were combined. Several stock solutions of this kind were prepared. They were stable for many months and were used to perform SERS experiments. (b) SERS Measurements. With most of the colloids prepared under different conditions of pH and different reactant concentrations, SERS spectra were recorded using nile blue A as a model compound. Our colloids, like most others, are stabilized against coagulation by electrostatic repulsive forces originating from anions adsorbed on the particle surface. With the pH being around 10, the stabilizing agent is most likely OH-, which adsorbes on the colloid surface and is less tightly bound than, for example, citrate in the colloids prepared according to the procedure of Lee and Meisel.12 This has two consequences. First, even small amounts of strongly adsorbing cationic analyte molecules can completely displace OH- and concomitantly establish a positive surface charge that, in turn, stabilizes the colloid electrostatically. Second, the analyte molecules produce good SERS spectra soon after addition to the colloid because the adsorption process is not delayed.
SERS spectra of nile blue A at different concentrations recorded with the colloid, whose absorption spectrum is displayed in Figure 1, are shown in Figure 3a. For comparison, the corresponding optical spectra are recorded in Figure 3b. The extinction coefficient of nile blue A at maximum absorbance is about 75 000 M-1 cm-1 but at 532 nm is only 8000 M-1 cm-1. At a high concentration of nile blue A, that is, 10-4 M, no SERS spectra could be recorded. For a 10-5 M solution, only a weak Raman signal was observed, although spectra with high signal intensity were produced with the colloid prepared with citrate. The signal intensity increased, however, with decreasing concentration of nile blue A and reached a maximum for a 10-7 M solution with the hydrazine colloid. A further decrease in the dye concentration resulted in a strong decrease of signal intensity. However, even with a 10-10 M solution, an interpretable spectrum could be recorded. With the colloid prepared by reduction of silver nitrate by citrate, the detection limit was 10-8 M, that is, at least 100-fold higher. In Figure 4a, the variation of the 1649 cm-1 band intensity with concentration of nile blue A is displayed. The data refer to spectra recorded 1 day after mixing the colloid with the dye. A similar dependence is observed for the data obtained from the spectra recorded only 1 min after carrying out the mixing process. In the latter spectra, the intensity was found to be lower by a factor of 2 at low nile blue A concentrations but larger at high concentrations under otherwise identical conditions. These differences are significant because the standard deviation of the signal intensity determined from many different samples is less than 5%. However, no kinetic experiments measuring the change of intensity with standing time have been carried out so far. For a given concentration, the intensity of the SERS signal also depends on the formal concentration of the silver in the hydrosol, that is, on the total number of particles. Figure 5 shows two examples. At low nile blue A concentration (10-7 M), the signal intensity increases
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Figure 3. (a) SERS spectra of nile blue A recorded with a silver colloid similar to that whose spectrum is displayed in Figure 1. The 17-day-old colloid had been mixed with the aqueous solution of the organic compound in a 1:1 ratio, i.e., the formal concentration of silver is 7.5 × 10-5 M. (b) Corresponding optical spectra.
Figure 4. Dependence of the SERS intensity on the concentration of nile blue A. (a) Experimental conditions similar to those chosen for the examples displayed in Figure 3, i.e., the final formal concentration of silver is 1.5 × 10-4 M, the final pH ) 10.2. The plotted intensity (measured 1 day after mixing the dye with the colloid) is the difference of the measured intensities at 1649 cm-1 (maximum intensity of the spectra displayed in Figure 3) and the 1574 cm-1 background. (b) The 3-day-old colloid was prepared by mixing equal volumes of 9 × 10-4 M silver nitrate and 1.12 × 10-3 M hydrazine dihydrochloride/NaOH (pH ) 10.8). The final formal concentration of silver is 4.5 × 10-4 M with a final pH of 7.1. The plotted intensity (recorded only 1 min after mixing the dye with the colloid) is the difference of the measured intensities at 1253 cm-1 (maximum intensity of the spectra displayed in Figure 3) and the 1226 cm-1 background.
Figure 5. Dependence of the SERS intensity on the silver colloid concentration. The experimental conditions for the graph with the open circles corresponds to those in Figure 4a, and those of the full squares correspond to those in Figure 4b.
strongly with increasing concentration of the colloid. But at high dye concentration (10-5 M), almost no Raman signal could be recorded independently of the particle concentration. Such a behavior was observed for all of the stable hydrazine colloids used so far, although a slight dependence of the signal intensity on the average particle size was found. In the examples presented in Figure 5, the average particle size was larger than in the example presented in Figure 4a because these colloids had been
prepared with more concentrated reactants. However, systematic experiments concerning the dependence of the SERS intensity on the particle size have not yet been carried out. The interplay between particle size and analyte concentration on one hand and SERS signal intensity on the other hand is rather complex due to several factors that determine the detectable flux of scattered photons. Provided that the same amount of elemental silver is present in the hydrosol, then the total surface area depends on the average size of the (spherical) particles. A higher proportion of smaller particles increases the total surface area and concomitantly the number of analyte molecules that can be adsorbed within the first monolayer. According to present models explaining the surface enhancement, it is mainly the molecules in the first layer that experience a large enhancement factor. Consequently, the SERS signal rises with concentration for as long as this concentration increase results in an increase in first-layer coverage. If the concentration is raised much above the level required for one monolayer and if additionally the adsorbed analyte molecules exhibit a high extinction coefficient at the wavelength of the probe laser and/or the Raman scattered light, then the SERS signal will be reduced due to absorption of either radiation by the analyte molecules. In our case, the dye nile blue A has at 532 nm an extinction coefficient of about 8000 M-1 cm-1, which
Silver Colloid as SERS Support
means that absorption is neglible only for solution concentrations less than 10-6 M. Another problem concerning absolute signal intensity is related to the dependence of plasmon resonance frequency (absorption maximum) on particle size and the dielectric constant of the immediate surroundings. It was claimed in the literature that the enhancement factor can vary significantly with particle size and/or the degree of particle aggregation. Whereas with monodisperse colloids, particle aggregation can easily be followed by the appearance of extinction spectra in the long wavelength range, in polydisperse systems the effect of aggregation is hard to distinguish from the effects of variable aspect ratios. The stability of the visible spectra with time (Figure 1) can be taken as evidence that the state of aggregation does not change on the long time scale if no analyte is present. Another point is that normally a small concentrated amount of the dye solution is added to the silver sol, so there is a high local dye concentration and no optimal intermixture can take place. One is not able to find a narrow maximum in SERS intensity. To avoid this problem, we add the dye to the colloid solution in a 1:1 volume ratio. Thus, we get optimal intermixture and no aggreation of the dye, and so we are able to find a relative sharp maximum in SERS intensity. The results obtained with our hydrazine colloids demonstrate that for a reliable quantification of organic compounds by SERS, low analyte concentrations should be used. However, the very high sensitivity shown in Figures 3 and 4a could only be obtained with colloids that were prepared with 1.5 × 10-4 M silver nitrate. With colloids prepared from a higher concentrated silver nitrate, no SERS spectra could be recorded if the dye concentration
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was lower than 10-7 M (see Figure 4b). However, a high signal intensity was obtained if the concentration exceeded this limit. This very sharp limit was found reproducibly in several experiments of this kind. The strong increase in signal intensity for concentrations around 10-7 M can therefore possibly allow a quantitative determination of an analyte by controlled dilution of a higher concentrated solution. Conclusions The experiments carried out so far show that a silver colloid prepared by reduction of silver nitrate with hydrazine in the described manner may offer a lot of opportunities in analytical chemistry. In contrast to most other reducing agents used for the preparation of silver colloids, the product of hydrazine oxidation, nitrogen, is inert and does not disturb or can be completely removed from the solution. Identification of organic compounds in extremely diluted aqueous solution should be possible without any up-concentration, provided that they exhibit a high affinity to the silver surface. Furthermore, the quantitative determination of specific families of organic compounds should be possible if no other compounds that can contribute a SERS signal are present. As the colloidal solution is not turbid, chemical reactions may simultaneously be followed photometrically and by means of Raman spectroscopy. Acknowledgment. Financial support by Deutsche Forschungsgemeinschaft and Fonds der Chemischen Industrie is gratefully acknowledged. LA000536Y
