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Time-dependent SERS of pseudoisocyanine on silver particles generated in silver bromide sol by laser illumination
Spccboehimiar AM, Vol. 49A, No. 2, pp. 167-172.1993 Printed in Great Britain
05l34-8539/93$6.tm+o.al @1993PWgMllOOPMSLtd
Time-dependent SERS of pseudoisocyapine on silver part&s in silver bromide sol by laser illumination IL KNIZIPP* and
H.
generated
KNHPP
Freie UniversitPt Berlin, Fachbcreich Physik, Amimallee 14, 1000 Berlin 33. Germany (Receiued 15 May 1992; in final form 25 June 1992; accepted 29 June 1992) Abatraet-SERS spectra of pseudoisocyanine dye (PIG) in silver bromide sob show a strong time-dependence. The enhancement factor, in general, follows the formation (and destruction) of SERSactive colloidal silver in the silver halide sob by laser illumination during the Raman measurement. Changes in the relative intensities within a characteristic line triplet of the SERS spectrum show that the surface potential which is “seen” by the dye molecules shifts to more Positive values with longer times. In particular, the values of the Potential hint at the existence of Ag, and A&. . . A& clusters as SERS-active adsorption sites of the dye molecules in AgBr SOIS.
INTRODUCTION
the last few years SERS has been observed not only on preformed and stable colloidal silver particles but also on silver halide sols [l-9]. In these systems, SERS-active silver particles (colloids and clusters) are generated in situ by laser illumination during the Raman scattering process. This results in a strong timedependence of the SERS spectra, which reflects the formation (and destruction) of the silver particles. Up to now it has not been exactly clear whether the appearance of SERS enhancement in silver halide sols is connected with the generation of electromagnetically SERS-active silver particles in a size range of about lO-lOOmn, or whether the formation of small silver clusters Ag, can give rise to SERS on silver halide sols due to a chemical SERS enhancement mechanism. Small silver particles in aqueous solution containing excess Ag+ ionsmay be regarded as tiny silver electrodes with an electrochemical standard potential which depends on the agglomeration number n or size [lo], i.e. SERS molecules on growing silver particles in silver halide sols “feel” not only changed electromagnetic enhancement due to changed particle size and shape, but also a changed electrochemical potential. SERS spectra can show a strong dependence on the potential of the SERS-active surface as has been demonstrated by SERS experiments on silver electrodes with varied potential [ll, 121. In this way, changes in the potential of growing silver particles can be monitored by changes in the SERS spectra of probe molecules. In Refs [7,8] we reported SERS of a cyanine dye in AgCl sols and photographic AgBr-emulsions. In AgCl sols, the time-dependence of the SERS intensity is correlated with the formation of an absorption due to colloidal silver in the silver halide sol. Changes in the relative intensities of some Raman bands of the dye in AgCl sols as well as in photographic AgBr emulsions were used to monitor changes to the surface potential of the SERS-active particles in these silver halide systems. In Ref. [9] changes in SERS intensity of pyridine in AgBr sols as a function of time are discussed within the framework of a changed surface potential of the small silver particles formed in the AgBr sol. In the following, the time-dependent SERS of pseudoisocyanine on a model AgBr sol will be reported. PIC seems to be a very favourable probe molecule for such investigations because the Raman spectra of this dye on electrodes with well-defined surface potentials have been extensively investigated [ll-141. The relative line intensities in the spectra show some characteristic changes which depend on the surface potential [ 11,131 [see Fig. l(b)]. OVER
*Author
to whom correspondence should be addressed. 167
168
K. I~EIPP
and H.
KNEW
There will be two observables in our experiment: (a) the general (electromagnetic) SERS enhancement factor which will be mainly determined by colloidal silver particles in the nm size range and which will be discussed in connection with the absorption spectra of the samples; and (b) the relative line intensities within a characteristic triplet in the Raman spectrum of PIC which will be compared with electrode SERS spectra at different defined potentials [ll-141. In this way, SERS enables us to estimate the surface potential of small silver particles in solution. Our experimental findings on AgBr sols will also be compared with SERS spectra of PIC on prefcrmed stable colloidal silver particles in different size ranges and with SERS measured with different excitation wavelengths, i.e. at monomer and J-aggregate resonance of the dye.
EXPERIMENTAL Raman spectra were measured in !IO“-scatteringgeometry with a conventional scanning Raman spectrometer. A 514 nm argon ion laser and a copper vapor laser with 510 and 578 nm wavelength (monomer and J-aggregate absorption) with an (average) laser power of about 200 mW were used
for excitation. The time required for registration of about lOOcm-’ (i.e. the spectral region which will be mainly considered in our discussionof time-dependent spectral changes)was about 1 min. The wavenumber accuracy of our measurements was +/-5 cm-‘. AgBr colloidal solutions were prepared by ‘mixing and shaking of equal volumes of 10d3molll aqueous KBr and AgN03 solutions [91* After about 30 mitt the sample became opaque indicating the formation of AgBr colloids [see curve 2 in Fig. 3(b)], and pseudoisocyanine in methanolic solution was then added. Additionally, preformed and stable silver colloids in different size ranges [see histograms in Fig. 3(b)] were used as SERS-active substrates. The Ag colloids were prepared from AgN03 and sodium citrate according to Ref. [ 151. Different conditions for the boiling procedure during the sol preparation resulted in changes of the sol morphology (~01s A, B, C). Particle size analysis was done by inspection of electron micrographs of the colloids. The dye concentration in all samples was about low6 mol/l.
RESULTSANDDISCUSSION
l,l’-Diethyl-2,2’-cyanine (PIC) is a very prominent dye molecule for studying enhanced Raman spectra of adsorbed molecules. Raman spectra have been measured on colloidal silver sols [5,7,16,17], on silver electrodes [ll-141, as well as on silver halides [5,7] including photographic relevant materials [8,18]. It can be concluded from Raman and visible absorption spectra [ll-14,16-191 that PIC can form J-aggregates, particularly in the adsorbed state. The observed Raman enhancement has been discussed in terms of normal molecular resonance Raman scattering, surface enhancement and aggregation enhancement due to J-aggregates of the dye molecules [14,12,19]. The relative intensities of the different lines in the Raman spectra of adsorbed PIC depend strongly on the experimental conditions, e.g. dye concentration, the solution pH [19], excitation wavelength and, in particular, on electrode potential [ll-141. This can be explained by the fact that the observed SERS spectra are the superposition of spectra dye to molecules in different chemical conditions (monomers and J-aggregates of transconformers and cis-conformers) [12, 191. The relative contributions from these three species depend on the experimental conditions. From this point-of-view the line triplet at 1356, 1370 and 139Ocm-’ is very instructive. The independent relative intensity variations of these three bands indicate that each is due to a dye molecule in a different structural conformation: 1356 and 1370cm-’ can be assigned to J-aggregates of transand ck- conformers, respectively; the 1390cm-’ line seems to be due to monomers [12]. In the following we shall restrict our considerations to these three Raman lines.
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Fig. 1. Tie-dependent SERS spectra of PIC on AgBr sol, 514 nm excitation, 200 mW, (b) PIC SERS spectra on a silver electrode at different surface potential (according to Ret%[ll, 131).
Figure l(a) shows SERS spectra of 10e6mol/l PIC on A~BF sol in the frequency region of interest after different times of 514nm laser irradiation. SERS spectra appear immediately after the beginning of the laser irradiation and show a small increase in scattering intensity in the first 5 min. Then a strong depletion of the SERS intensity starts to take place and after about 10 min the SERS intensity is at the level of Raman intensity of lO-‘jmol/l PIC in water (Fig. 3 shows for comparison a normal Raman spectrum of PIC in water in 10m5mol/l concentration). This time-dependent behaviour of the SERS intensities can be understood by a comparison of the SERS spectra and the time-dependent absorption spectra of the sample in Fig. 2. Curve 1 shows the absorption of the sample immediately after mixing AgNO, and KElr solutions. Curve 2 represents the sample after about 30 min and shows a strong increase in absorption in the W region due to the AgRr absorption at about 320 nm. Curve 3 which was measured after PIC addition and 1 min laser illumination shows an increase in the absorption between 400 and 6Wnm which indicates the formation of colloidal SERS-active silver. The curve also shows the PIC monomer
vvbvelength hm)
Fig. 2. Absorption spectra of AgBr solution at different times of laser illumination (514nm, 200 mW): 1, KBr and AgNOs solution immediately after mixing; 2, AgBr colloidal solution; 3, A@ colloidal solution with 10-6mol/l PIC after 1 min laser illumination; 4, AgBr colloidal solution with 10-6mol/l PIC after 4 min laser illumination; 5, AgBr colloidal solution with 10m6molll PIC after 10 min laser ilhunination.
K.
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500
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Fig. 3. (a) SERS of 10m6molll PIC on different silver sols A, B and C at 514 nm excitation, normal Raman scattering’of 10m5molll PIC in Hz0 (the spectra of sol C are reduced by a factor of 20, and the spectra of sol B are reduced by a factor of 4); (b) histograms of sols A, B and C; (c) absorption spectra of sols A, B and C.
absorption at about 510 nm, the blue shifted dimer absorption and also a small absorption at about 580 nm due to J-aggregates of the dye. The colloidal absorption increases further during the next few minutes (see curve 4). However, at longer times the colloidal particles coagulate and after about 10 min very large silver particles can be observed. This results in a strong decrease of the colloidal absorption spectra of the sample (see curve 5). The time-dependent intensity of SERS reflects this formation and destruction of colloidal silver. Whereas the SERS intensity in the first few minutes increases, later it shows a strong depletion in accordance with the depletion of the colloidal absorption. The relative intensities of the triplet also show characteristic time-dependent changes. Whereas in the first few minutes the 1356 cm-’ line was most intense in the triplet, some minutes later the relative intensities change and the line at 1370 cm-’ became the most intense one. Similar changes in the relative intensities have also been observed on silver electrodes as a function of the surface potential. Figure l(b) shows SERS spectra of PIC on silver electrodes with well-defined surface potentials as reported in Refs [ll, 131. Surface potential on the colloids can be estimated from a comparison of SERS spectra with these electrode SERS spectra at 488 nm excitation. In this way, the potential of the colloidal particles in the first few minutes seems to be more negative than -1.3 V vs SCE. The spectrum after 5.5 min can be compared with electrode SERS spectra at a surface potential of about -0.9--0.7 V vs SCE. This is the surface potential which we obtained in most of our stable silver sols with a particle size between 40 and 150 nm. Figure 3(a) shows SERS spectra in the frequency range of interest measured at three different silver sols with 514 nm excitation. The different SERS intensities can be explained by the different electromagnetic enhancement of the three different ~01s. Figure 3(b) and (c) show the size distribution of the particles in sols A, B and C and their absorption spectra, respectively. The enhancement factor increases from sol A to sol C. This is in accordance with the fact that there is an optimum particle size range for
Time-dependent SERS of pseudoisocyanine
171
electromagnetic SERS enhancement, which for silver particles is in the range of about 100 nm [20]. In sol A we obtained with I,,, > &, a spectrum similar to the AgBr sol SERS in the first minutes, which hints at very negative potentials. Sols B and C show SERS spectra similar to SERS spectra on AgBr sol after about indicate surface potentials 5 min laser illumination. The relative line intensities I I370> Z13% more positive than -0.9 V vs SCE. The SERS spectra on the stable silver colloids in Fig. 3 did not show any changes in the relative intensities with time [16]. The experimental results of Ref. [12] lead to the interpretation that an increased proportion of the dye is J-aggregated with decreased potential and that a very negative potential obviously increases the number of tram-trans intermolecular associations in the J-aggregates to which the 1356 cm-’ band had been assigned. The interpretation is supported by our experimental results with two different excitation wavelengths, i.e. at the monomer and J-aggregate resonance. Figure 4 shows SERS spectra of PIC on sol C with 510 and 578 nm excitation. As expected, the spectrum with 510 nm excitation is in accordance with the 514 nm-excited spectrum in Fig. 3. With excitation at 578 nm into the J-aggregate resonance, the 1356cm-’ line becomes dominant, i.e. the spectrum is similar to SERS spectra on sol A or on AgBr sol in the first minutes. In other words, with J-aggregate resonance excitation we could reach the same SERS spectra as for a very negative surface potential which also favours J-aggregated dye molecules. The very negative surface potentials estimated from a comparison with electrode spectra can hardly be explained by colloidal particles in run size range. From Fig. 1 in Ref. [lo] it can be concluded that the potential of such particles is much more positive and strives towards +0.799 V vs NHE (or +0.55 V vs SCE, respectively). Potentials between -0.7 V vs SCE and - 1.3 V vs SCE and more negative as we have estimated can be explained by small silver clusters Ag. (n 5 6) (see below and Ref. [lo]). In this way, our measurements give strong hints to the existence of small silver clusters in the AgBr sols after laser irradiation and their important role as SERS-active adsorption sites for the dye molecules [21,22]. The colloidal silver “only” gives rise to an electromagnetic enhancement. This electromagnetic enhancement mechanism does not demand an adsorption of the Raman molecule at the colloidal particles and gives contributions for molecules in the vicinity of the colloids, too. A more detailed comparison of our experimental results with the standard redox potential of a silver microelectrode given in Ref. [lo] suggests that the potential smaller than -1.3 V vs SCE which we observed in AgBr sol in the first minutes could be due to single silver atoms. A potential of about -2 V vs SCE was calculated for these free silver
x45
I xL=510nm
A,= 578 nm
Fig. 4. SERS of PIC on sol C at monomer (SlOnm)- and I-aggregate (578 nm)-resonance excitation (the 578 nm spectrum is reduced by a factor of 45).
172
K.
KNEW
and H.
KNFJIPP
atoms in solution in Ref. [lo]. According to Ref. [lo], potentials between -0.7 V vs SCE and -0.9 V vs SCE which we observed later can be explained by clusters of Ag,, Ag, or Aga. With respect to very similar SERS spectra of PIC in this potential range between -0.7 and -0.9 V vs SCE [ll, 121 we cannot distinguish between these three clusters. We were unable to find at any time SERS spectra similar to characteristic electrode SERS spectra at -1.0 V vs SCE [see Fig. l(b)], a potential which could be assigned to A& clusters. Additionaly, we did not observe SERS spectra at a relatively positive surface potential (about -0.3 V vs SCE) which was calculated in Ref. [lo] for A& clusters. In summary, our experimental results suggest the existence of small silver clusters as SERS-active adsorption sites in AgBr sols in addition to silver particles in the nanometer size range which give rise to an electromagnetic enhancement. In particular, it can be concluded that there are Agi and Ag,, clusters (12=4, 5, 6). However, our SERS experiments gave no hints at the existence of Agz and A& clusters. Our experimental data fit well with the results of Refs [21, 221 where it was defined that clusters, playing a role in SERS, are in the form of Ag,, (3
REFERENCES [l] [2] [3] [4] [5] [6] [7] [8] [9] [lo] [ll] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
X. Gao, Ch. Wan, T. He, J. Li, H. Xin and F. Liu, Chem. Phys. Lett. 112,465 (1984). W. Jian, L. Dawei, X. Houwen, S. Xu and L. Far&hen, Spectrochim. Acta 43A, 375 (1987). L. Dawei, W. Jian, X. Houwen, S. Xu and L. Fan-Chen, Spectrochim. Acta 43A, 379 (1987). A. M. Ahem and R. L. Garrell, And. Chem. 59,2813 (1987). J. Wang, P. Zhang, T. He, H. Xin and F. Liu, 1. Phys. &ma. 92, 1942 (1988). F. H. Haegel and A. Wokaun, Chem. Phys. Len. 157,328 (1989). K. Kneipp, 1. Molec. Struct. 218, 357 (1990). K. Kneipp, W. Jahr and G. Roewer, C&m. Phys. Len. 163,105 (1989). Z. Hongguang, X. Houwen, H. Tiangjing and L. Fanchen, Spectrochim. Acta 47A, 927 (1991). A. Henglein, Ber. Bunsenges. Phys. Chem. 94,600 (1990). X. Li, B. Gu and D. L. Akins, Gem. Phys. Lett. 105,263 (1984). D. L. Akins, J. W. Mach and H. R. Zhu, J. Phys. Chem. 95,793 (1991). B. Gu and D. L. Akins, Chem. Phys. Lett. 113,558 (1985). D. L. Akins, C. K. Akpabli and X. Li, J. Phys. C/rem. 93,1977 (1989). P. C. Lee and D. Meisel, J. PhyJ. Chem. 86,339l (1982). D. L. Akins, J. Colfoid In#e?face Sci. 98, 373 (1982). K. Kneipp, H. Kneipp and M. Rentach, J. Molec. Struct. 156.331 (1987). E. S. Brandt, Appl. Spectrosc. 42,882 (1988). D. L. Akins and J. W. Ma&in, J. Phys. Chem. 93,5999 (1989). A. Wokaun, J. P. Gordon and P. F. Liao, Phys. Reu. Mt. 48,957 (1982). D. Roy and T. E. Furtak, Chem. Phys. Lett. 124,299 (1986). S. Y. Dong, G. Wang, W. Wang, Z. Zhang and J. Zheng, Appl. Phys. B49.553 (1989).
05l34-8539/93$6.tm+o.al @1993PWgMllOOPMSLtd
Time-dependent SERS of pseudoisocyapine on silver part&s in silver bromide sol by laser illumination IL KNIZIPP* and
H.
generated
KNHPP
Freie UniversitPt Berlin, Fachbcreich Physik, Amimallee 14, 1000 Berlin 33. Germany (Receiued 15 May 1992; in final form 25 June 1992; accepted 29 June 1992) Abatraet-SERS spectra of pseudoisocyanine dye (PIG) in silver bromide sob show a strong time-dependence. The enhancement factor, in general, follows the formation (and destruction) of SERSactive colloidal silver in the silver halide sob by laser illumination during the Raman measurement. Changes in the relative intensities within a characteristic line triplet of the SERS spectrum show that the surface potential which is “seen” by the dye molecules shifts to more Positive values with longer times. In particular, the values of the Potential hint at the existence of Ag, and A&. . . A& clusters as SERS-active adsorption sites of the dye molecules in AgBr SOIS.
INTRODUCTION
the last few years SERS has been observed not only on preformed and stable colloidal silver particles but also on silver halide sols [l-9]. In these systems, SERS-active silver particles (colloids and clusters) are generated in situ by laser illumination during the Raman scattering process. This results in a strong timedependence of the SERS spectra, which reflects the formation (and destruction) of the silver particles. Up to now it has not been exactly clear whether the appearance of SERS enhancement in silver halide sols is connected with the generation of electromagnetically SERS-active silver particles in a size range of about lO-lOOmn, or whether the formation of small silver clusters Ag, can give rise to SERS on silver halide sols due to a chemical SERS enhancement mechanism. Small silver particles in aqueous solution containing excess Ag+ ionsmay be regarded as tiny silver electrodes with an electrochemical standard potential which depends on the agglomeration number n or size [lo], i.e. SERS molecules on growing silver particles in silver halide sols “feel” not only changed electromagnetic enhancement due to changed particle size and shape, but also a changed electrochemical potential. SERS spectra can show a strong dependence on the potential of the SERS-active surface as has been demonstrated by SERS experiments on silver electrodes with varied potential [ll, 121. In this way, changes in the potential of growing silver particles can be monitored by changes in the SERS spectra of probe molecules. In Refs [7,8] we reported SERS of a cyanine dye in AgCl sols and photographic AgBr-emulsions. In AgCl sols, the time-dependence of the SERS intensity is correlated with the formation of an absorption due to colloidal silver in the silver halide sol. Changes in the relative intensities of some Raman bands of the dye in AgCl sols as well as in photographic AgBr emulsions were used to monitor changes to the surface potential of the SERS-active particles in these silver halide systems. In Ref. [9] changes in SERS intensity of pyridine in AgBr sols as a function of time are discussed within the framework of a changed surface potential of the small silver particles formed in the AgBr sol. In the following, the time-dependent SERS of pseudoisocyanine on a model AgBr sol will be reported. PIC seems to be a very favourable probe molecule for such investigations because the Raman spectra of this dye on electrodes with well-defined surface potentials have been extensively investigated [ll-141. The relative line intensities in the spectra show some characteristic changes which depend on the surface potential [ 11,131 [see Fig. l(b)]. OVER
*Author
to whom correspondence should be addressed. 167
168
K. I~EIPP
and H.
KNEW
There will be two observables in our experiment: (a) the general (electromagnetic) SERS enhancement factor which will be mainly determined by colloidal silver particles in the nm size range and which will be discussed in connection with the absorption spectra of the samples; and (b) the relative line intensities within a characteristic triplet in the Raman spectrum of PIC which will be compared with electrode SERS spectra at different defined potentials [ll-141. In this way, SERS enables us to estimate the surface potential of small silver particles in solution. Our experimental findings on AgBr sols will also be compared with SERS spectra of PIC on prefcrmed stable colloidal silver particles in different size ranges and with SERS measured with different excitation wavelengths, i.e. at monomer and J-aggregate resonance of the dye.
EXPERIMENTAL Raman spectra were measured in !IO“-scatteringgeometry with a conventional scanning Raman spectrometer. A 514 nm argon ion laser and a copper vapor laser with 510 and 578 nm wavelength (monomer and J-aggregate absorption) with an (average) laser power of about 200 mW were used
for excitation. The time required for registration of about lOOcm-’ (i.e. the spectral region which will be mainly considered in our discussionof time-dependent spectral changes)was about 1 min. The wavenumber accuracy of our measurements was +/-5 cm-‘. AgBr colloidal solutions were prepared by ‘mixing and shaking of equal volumes of 10d3molll aqueous KBr and AgN03 solutions [91* After about 30 mitt the sample became opaque indicating the formation of AgBr colloids [see curve 2 in Fig. 3(b)], and pseudoisocyanine in methanolic solution was then added. Additionally, preformed and stable silver colloids in different size ranges [see histograms in Fig. 3(b)] were used as SERS-active substrates. The Ag colloids were prepared from AgN03 and sodium citrate according to Ref. [ 151. Different conditions for the boiling procedure during the sol preparation resulted in changes of the sol morphology (~01s A, B, C). Particle size analysis was done by inspection of electron micrographs of the colloids. The dye concentration in all samples was about low6 mol/l.
RESULTSANDDISCUSSION
l,l’-Diethyl-2,2’-cyanine (PIC) is a very prominent dye molecule for studying enhanced Raman spectra of adsorbed molecules. Raman spectra have been measured on colloidal silver sols [5,7,16,17], on silver electrodes [ll-141, as well as on silver halides [5,7] including photographic relevant materials [8,18]. It can be concluded from Raman and visible absorption spectra [ll-14,16-191 that PIC can form J-aggregates, particularly in the adsorbed state. The observed Raman enhancement has been discussed in terms of normal molecular resonance Raman scattering, surface enhancement and aggregation enhancement due to J-aggregates of the dye molecules [14,12,19]. The relative intensities of the different lines in the Raman spectra of adsorbed PIC depend strongly on the experimental conditions, e.g. dye concentration, the solution pH [19], excitation wavelength and, in particular, on electrode potential [ll-141. This can be explained by the fact that the observed SERS spectra are the superposition of spectra dye to molecules in different chemical conditions (monomers and J-aggregates of transconformers and cis-conformers) [12, 191. The relative contributions from these three species depend on the experimental conditions. From this point-of-view the line triplet at 1356, 1370 and 139Ocm-’ is very instructive. The independent relative intensity variations of these three bands indicate that each is due to a dye molecule in a different structural conformation: 1356 and 1370cm-’ can be assigned to J-aggregates of transand ck- conformers, respectively; the 1390cm-’ line seems to be due to monomers [12]. In the following we shall restrict our considerations to these three Raman lines.
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169
Time-dependent SERS of pseudoisocyanine
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Fig. 1. Tie-dependent SERS spectra of PIC on AgBr sol, 514 nm excitation, 200 mW, (b) PIC SERS spectra on a silver electrode at different surface potential (according to Ret%[ll, 131).
Figure l(a) shows SERS spectra of 10e6mol/l PIC on A~BF sol in the frequency region of interest after different times of 514nm laser irradiation. SERS spectra appear immediately after the beginning of the laser irradiation and show a small increase in scattering intensity in the first 5 min. Then a strong depletion of the SERS intensity starts to take place and after about 10 min the SERS intensity is at the level of Raman intensity of lO-‘jmol/l PIC in water (Fig. 3 shows for comparison a normal Raman spectrum of PIC in water in 10m5mol/l concentration). This time-dependent behaviour of the SERS intensities can be understood by a comparison of the SERS spectra and the time-dependent absorption spectra of the sample in Fig. 2. Curve 1 shows the absorption of the sample immediately after mixing AgNO, and KElr solutions. Curve 2 represents the sample after about 30 min and shows a strong increase in absorption in the W region due to the AgRr absorption at about 320 nm. Curve 3 which was measured after PIC addition and 1 min laser illumination shows an increase in the absorption between 400 and 6Wnm which indicates the formation of colloidal SERS-active silver. The curve also shows the PIC monomer
vvbvelength hm)
Fig. 2. Absorption spectra of AgBr solution at different times of laser illumination (514nm, 200 mW): 1, KBr and AgNOs solution immediately after mixing; 2, AgBr colloidal solution; 3, A@ colloidal solution with 10-6mol/l PIC after 1 min laser illumination; 4, AgBr colloidal solution with 10-6mol/l PIC after 4 min laser illumination; 5, AgBr colloidal solution with 10m6molll PIC after 10 min laser ilhunination.
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.
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. 400
500
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600
(nm)
Fig. 3. (a) SERS of 10m6molll PIC on different silver sols A, B and C at 514 nm excitation, normal Raman scattering’of 10m5molll PIC in Hz0 (the spectra of sol C are reduced by a factor of 20, and the spectra of sol B are reduced by a factor of 4); (b) histograms of sols A, B and C; (c) absorption spectra of sols A, B and C.
absorption at about 510 nm, the blue shifted dimer absorption and also a small absorption at about 580 nm due to J-aggregates of the dye. The colloidal absorption increases further during the next few minutes (see curve 4). However, at longer times the colloidal particles coagulate and after about 10 min very large silver particles can be observed. This results in a strong decrease of the colloidal absorption spectra of the sample (see curve 5). The time-dependent intensity of SERS reflects this formation and destruction of colloidal silver. Whereas the SERS intensity in the first few minutes increases, later it shows a strong depletion in accordance with the depletion of the colloidal absorption. The relative intensities of the triplet also show characteristic time-dependent changes. Whereas in the first few minutes the 1356 cm-’ line was most intense in the triplet, some minutes later the relative intensities change and the line at 1370 cm-’ became the most intense one. Similar changes in the relative intensities have also been observed on silver electrodes as a function of the surface potential. Figure l(b) shows SERS spectra of PIC on silver electrodes with well-defined surface potentials as reported in Refs [ll, 131. Surface potential on the colloids can be estimated from a comparison of SERS spectra with these electrode SERS spectra at 488 nm excitation. In this way, the potential of the colloidal particles in the first few minutes seems to be more negative than -1.3 V vs SCE. The spectrum after 5.5 min can be compared with electrode SERS spectra at a surface potential of about -0.9--0.7 V vs SCE. This is the surface potential which we obtained in most of our stable silver sols with a particle size between 40 and 150 nm. Figure 3(a) shows SERS spectra in the frequency range of interest measured at three different silver sols with 514 nm excitation. The different SERS intensities can be explained by the different electromagnetic enhancement of the three different ~01s. Figure 3(b) and (c) show the size distribution of the particles in sols A, B and C and their absorption spectra, respectively. The enhancement factor increases from sol A to sol C. This is in accordance with the fact that there is an optimum particle size range for
Time-dependent SERS of pseudoisocyanine
171
electromagnetic SERS enhancement, which for silver particles is in the range of about 100 nm [20]. In sol A we obtained with I,,, > &, a spectrum similar to the AgBr sol SERS in the first minutes, which hints at very negative potentials. Sols B and C show SERS spectra similar to SERS spectra on AgBr sol after about indicate surface potentials 5 min laser illumination. The relative line intensities I I370> Z13% more positive than -0.9 V vs SCE. The SERS spectra on the stable silver colloids in Fig. 3 did not show any changes in the relative intensities with time [16]. The experimental results of Ref. [12] lead to the interpretation that an increased proportion of the dye is J-aggregated with decreased potential and that a very negative potential obviously increases the number of tram-trans intermolecular associations in the J-aggregates to which the 1356 cm-’ band had been assigned. The interpretation is supported by our experimental results with two different excitation wavelengths, i.e. at the monomer and J-aggregate resonance. Figure 4 shows SERS spectra of PIC on sol C with 510 and 578 nm excitation. As expected, the spectrum with 510 nm excitation is in accordance with the 514 nm-excited spectrum in Fig. 3. With excitation at 578 nm into the J-aggregate resonance, the 1356cm-’ line becomes dominant, i.e. the spectrum is similar to SERS spectra on sol A or on AgBr sol in the first minutes. In other words, with J-aggregate resonance excitation we could reach the same SERS spectra as for a very negative surface potential which also favours J-aggregated dye molecules. The very negative surface potentials estimated from a comparison with electrode spectra can hardly be explained by colloidal particles in run size range. From Fig. 1 in Ref. [lo] it can be concluded that the potential of such particles is much more positive and strives towards +0.799 V vs NHE (or +0.55 V vs SCE, respectively). Potentials between -0.7 V vs SCE and - 1.3 V vs SCE and more negative as we have estimated can be explained by small silver clusters Ag. (n 5 6) (see below and Ref. [lo]). In this way, our measurements give strong hints to the existence of small silver clusters in the AgBr sols after laser irradiation and their important role as SERS-active adsorption sites for the dye molecules [21,22]. The colloidal silver “only” gives rise to an electromagnetic enhancement. This electromagnetic enhancement mechanism does not demand an adsorption of the Raman molecule at the colloidal particles and gives contributions for molecules in the vicinity of the colloids, too. A more detailed comparison of our experimental results with the standard redox potential of a silver microelectrode given in Ref. [lo] suggests that the potential smaller than -1.3 V vs SCE which we observed in AgBr sol in the first minutes could be due to single silver atoms. A potential of about -2 V vs SCE was calculated for these free silver
x45
I xL=510nm
A,= 578 nm
Fig. 4. SERS of PIC on sol C at monomer (SlOnm)- and I-aggregate (578 nm)-resonance excitation (the 578 nm spectrum is reduced by a factor of 45).
172
K.
KNEW
and H.
KNFJIPP
atoms in solution in Ref. [lo]. According to Ref. [lo], potentials between -0.7 V vs SCE and -0.9 V vs SCE which we observed later can be explained by clusters of Ag,, Ag, or Aga. With respect to very similar SERS spectra of PIC in this potential range between -0.7 and -0.9 V vs SCE [ll, 121 we cannot distinguish between these three clusters. We were unable to find at any time SERS spectra similar to characteristic electrode SERS spectra at -1.0 V vs SCE [see Fig. l(b)], a potential which could be assigned to A& clusters. Additionaly, we did not observe SERS spectra at a relatively positive surface potential (about -0.3 V vs SCE) which was calculated in Ref. [lo] for A& clusters. In summary, our experimental results suggest the existence of small silver clusters as SERS-active adsorption sites in AgBr sols in addition to silver particles in the nanometer size range which give rise to an electromagnetic enhancement. In particular, it can be concluded that there are Agi and Ag,, clusters (12=4, 5, 6). However, our SERS experiments gave no hints at the existence of Agz and A& clusters. Our experimental data fit well with the results of Refs [21, 221 where it was defined that clusters, playing a role in SERS, are in the form of Ag,, (3
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