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Surface-enhanced raman scattering of polymethine dyes on silver colloidal particles
Vohxne 99, number 5,6
RAMAN SCA‘ITERING
SU~ACE-E~C~R OF POL~~~~
19 August 1983
CHEMKXL PHYSICS LETTERS
DYES ON SILVER COLLOIDAL PARTICLES
K. KNEIPP, G. HINZMANN and D. FASSLER Chemical Department. Fn~edrichSchilIer-University. DDR-6900 Jena. German Democratic Republic
Received 26 April 1983; in final form 13 June 1983
SERS was observed from adsrbed cationic cyanine and neutral merocyanine dyes, whereas the tested osonol dye did not show any adsorption on the silver colioids. A comparison between Raman scattering of the first three representativesof the homologous series of one dye and Raman experiments performed on the same dyes in solution confnms the e_xistence of SERS.
1..Introduction After the discovery of surfaceenhanced Raman scattering on pyridine on silver electrodes [ 1 J a similar effect was obtained on silver and gold sols [2]. In the following years several molecules were studied with regard to their adsorption behaviour on silver and gold colloids and enhanced Raman scattering [3-6] including hyper-Raman scattering (73 _ Investigations of Raman scattering from molecules adsorbed on metal colloids have a basic aim in understanding the enhancing effect and in studying the interaction between molecule and metal_ This question could be relevant for many fields of surface physics and chemistry,particularly for investigations ofphotographic processes and spectral sensitization. Additionally SERS is of interest as a spectral analytic method because of the possibility to realize vibrational speetroscopic investigations in concentration ranges smaller than 10F7 mol/Q_ With regard to these principal possibilities of the method, we report SERS investigations on several polymethme dyes, a class of dyes which is of importance for spectral sensitization in photography_
Its absorption maximum was at -420 nm, the concentration was x I Oe3 mol/Q. This sol was mixed with the 10-5-10-8 molfQ aqueous dye solutions_ The experimental apparatus for Raman spectroscopy consisted of an Ar* ion laser (type ILA 13-O-1, VEB Carl Zeiss Jena), double grating monochromator, photomultiplier tube and lock-in amplifier. The absorption spectra were recorded on specord M 40 (VEB Carl Zeiss Jena).
3_ Resultsand discussion The following dyes were studied (H denotes a lonepair of electrons on the nitrogen atom), with the absorption maximum in methanol indicated:
557
nm
2. Experimental The silver sol was prepared from AgNO3 and sodium citrate according to a procedure described in ref. [S] _ 0 009-2614/83/0000-0000/$03.00
0 1983 North-Holland
CP 655 ran 503
Volume 99, number 5.6
CHEMICAL PHYSICS LETTERS
D ?-&zy I CH3
E
468
nm
H3c~cH3 Q
Q
S03H
544 =
SO3H
;Ul dyes were investigated in concentration ranges where no aggregation was obtained even in aqueous solution. which favours the dimerisation of these dyes LS]Apart from the oaonol (dye E), the dyes investigdtcd adsorbed on the silver colloids. This result is contrary to rei‘. 151, where negatively charged dyes adsorbed on silver sols and esperiments with cationic dyes were not successful. Fig. 1 shows for example the absorption spectra of the sol, the aqueous solution of the dye Cand the 1 : 1 mixture of dye solution and silver sol, respectiveIy.
ir-
-------f
19 AuFst 1983
The absorption spectrum of the mixture clearly shows a broadening in compririson with the pure sol_ in ref. [7] this effect was also observed and possible mechanisms for it were discussed. Our experiments thus far performed cannot give a sure explanation for the broadening obtained on the addition of all dyes to the SOL
Fig. 2 shows the Raman spectra of the first three representatives of the homologeous series of 3,3’diet?lyl-2,2’-benzthiacyanines (dyes A, B, C) adsorbed on silver colloids. In ref. [S] these three dyes were investigated in 5 X 10-s -5 X 10-s mol/Q methanolic and methanolic aqueous or aqueous solutions, respectively. In those esperiments in solutions different optimum excitation wavelengths were necessary to get Raman spectra due to the different absorption max-
ima of the dyes. Because of their strong fluorescence and the small photo stability, some dyes could be investigated by CARS only [S] _As shown in Gg_ 2, the same dyes adsorbed on silver colloids gave rise to easily observable Raman spectra in concentration ranges 1O-3 -IO-” times smaller. A further advantage of Raman investigations of polyrnethine
dyes adsorbed on silver colloids in com-
parison with dye solutions is the suppression of fluorescence and a better photo stability. Contrary to the experiments performed in solutions, the different adsorbed dyes of the ~1onloIogeous series could be investigated by the same excitation wavelength (5 13.5 nm, see fig. 3), in spite of their different absorption maxima. This gives hints that the enhancement
Fi& 1. Absorptmn spectra of 0.5 x IO9 mol/P silver sol (1); 1 : 1 mi\ture of 10e3 mol/S silver sol with 1.1 X lo6 mol/P .~queousdye C soturion (1) and 0.7 x IO* mol/P aqueous solurion of dpe C (3). Optical path for (l)-(3) t\as 0.2 cm.
upon adsorption on the metal for the different dyes must be different or that for adsorbed dye molecules the resonance enhancement due to the specific molecular absorption, which was necessary for successful Raman scattering in solutions, is not essential. We did not find for the investigated dyes a zero surface enhancement for dyes as discussed [63 _ A detaiIed discussion of the scattering behaviour of the homologous series (dyes A, B, C) in the light of models describing SERS will be given in a separate publication_ Fig. 3 shows the Raman spectra of merocyanine (dye D) in methanolic solution (a) and adsorbed on silver colloids (b). Also this dye shows an essential growth of Raman scattering intensity after adsorption (see experimental conditions given in fig. 3). The frequency differences between cyanine dyes in solution [8] and adsorbed on silver colloids (*IO cm-l)
CHEMICAiPHYSICS
Volume 99, number 5,6
a
I’
5
19 Au-gist-3983
iETTERS
b m
B
I
1400 f Lfye:
3406 I Oy*:
c
too0 t
606
ZOD
I
mm f
600
201
13
Fii. 2. Raman spectra of dye A {a), dye B fb) and dye C (c) adsorbed on silver sol from lCJ_? molfn aqueous solution at nearly the same esperimental conditions.
Volume 99.
CHEMJCAL PHYSICS LETTERS
number 5,6
19 August 1983
b
lb00 Dye:
1000
600
1400
ZOO
I
Dye:
D
1000
I
4. Conclusions
SERS on silver colloids is shown to be a powerful and simple method for studying vibrational spectra of polymethine dyes. Some methodolo@al problems connected with Raman spectroscopic investigations of these molecules were overcome_ Potential sensitizing dyes especially seem to have rather good adsorption properties and efficient SERS. In this way vibrational spectroscopic investigations on these dyes could be easily performed, resulting in additional information about sensitizing molecules and spectral sensitization. 506
200
D
Fip. 3. Raman spectra of dye D in Z X 10v3 methanolic solution (a) and of 10m7 mol/P aqueoussohrtuon sot (b): Botb spectra v.ere registmted with the same expcrinrental conditions.
are of the sdtne order observed on changes in the solvent 191. As expected for the dipolar merocyanine, the frequency shifts between dissolved and adsorbed molecules are more evident- A discussion of the frequency shiftingand intensity behaviour of the different Raman lines will be given later.
600 t
mixed I : 1 with silver
References [ 11 DJ. Jeanmaire and R.P. van Duyne, J. Electroam& 64 (1977) [ 2]
Chem.
1.
J-A. Creighton, CC. Blatchford and hi.G. Albrecht, J.
Chem. Sot. Faraday Trans. II 75 (1979) 970. [3] A-V. Baranov and Y-S. Bobowitsch. Opt. Spectry. 52 (1982) 385. ]4] A.V_ Baranov and Y-S. Bobowitsch, Soviet Phys. JETP Letters 35 (1982) 149. [S] PC. Lee and D. Meisel, J. Phys. Chem. 86 (1982) 3391. [ 61 A. Bachackashvilh. S. Efrima, 8. Katz and 2_ Priel, Chem. Phys. Letters 94 (1983) 571. [ 71 A.V. Baranov and Y.S. Bobovitsch, Soviet Phys. JETP Letters 36 (1982) 277. IS] A- Lau, K_ Kneipp, W_ Werncke, K_ Lenz, HJ_ Weigmann, D. Fassler and G. Hinzmann, Advan. Mol. Relaxation Interaction Processes 24 (19S2) 27. [9] G. Hinzmarm K. Kneipp and D. Fassier, to be published.
RAMAN SCA‘ITERING
SU~ACE-E~C~R OF POL~~~~
19 August 1983
CHEMKXL PHYSICS LETTERS
DYES ON SILVER COLLOIDAL PARTICLES
K. KNEIPP, G. HINZMANN and D. FASSLER Chemical Department. Fn~edrichSchilIer-University. DDR-6900 Jena. German Democratic Republic
Received 26 April 1983; in final form 13 June 1983
SERS was observed from adsrbed cationic cyanine and neutral merocyanine dyes, whereas the tested osonol dye did not show any adsorption on the silver colioids. A comparison between Raman scattering of the first three representativesof the homologous series of one dye and Raman experiments performed on the same dyes in solution confnms the e_xistence of SERS.
1..Introduction After the discovery of surfaceenhanced Raman scattering on pyridine on silver electrodes [ 1 J a similar effect was obtained on silver and gold sols [2]. In the following years several molecules were studied with regard to their adsorption behaviour on silver and gold colloids and enhanced Raman scattering [3-6] including hyper-Raman scattering (73 _ Investigations of Raman scattering from molecules adsorbed on metal colloids have a basic aim in understanding the enhancing effect and in studying the interaction between molecule and metal_ This question could be relevant for many fields of surface physics and chemistry,particularly for investigations ofphotographic processes and spectral sensitization. Additionally SERS is of interest as a spectral analytic method because of the possibility to realize vibrational speetroscopic investigations in concentration ranges smaller than 10F7 mol/Q_ With regard to these principal possibilities of the method, we report SERS investigations on several polymethme dyes, a class of dyes which is of importance for spectral sensitization in photography_
Its absorption maximum was at -420 nm, the concentration was x I Oe3 mol/Q. This sol was mixed with the 10-5-10-8 molfQ aqueous dye solutions_ The experimental apparatus for Raman spectroscopy consisted of an Ar* ion laser (type ILA 13-O-1, VEB Carl Zeiss Jena), double grating monochromator, photomultiplier tube and lock-in amplifier. The absorption spectra were recorded on specord M 40 (VEB Carl Zeiss Jena).
3_ Resultsand discussion The following dyes were studied (H denotes a lonepair of electrons on the nitrogen atom), with the absorption maximum in methanol indicated:
557
nm
2. Experimental The silver sol was prepared from AgNO3 and sodium citrate according to a procedure described in ref. [S] _ 0 009-2614/83/0000-0000/$03.00
0 1983 North-Holland
CP 655 ran 503
Volume 99, number 5.6
CHEMICAL PHYSICS LETTERS
D ?-&zy I CH3
E
468
nm
H3c~cH3 Q
Q
S03H
544 =
SO3H
;Ul dyes were investigated in concentration ranges where no aggregation was obtained even in aqueous solution. which favours the dimerisation of these dyes LS]Apart from the oaonol (dye E), the dyes investigdtcd adsorbed on the silver colloids. This result is contrary to rei‘. 151, where negatively charged dyes adsorbed on silver sols and esperiments with cationic dyes were not successful. Fig. 1 shows for example the absorption spectra of the sol, the aqueous solution of the dye Cand the 1 : 1 mixture of dye solution and silver sol, respectiveIy.
ir-
-------f
19 AuFst 1983
The absorption spectrum of the mixture clearly shows a broadening in compririson with the pure sol_ in ref. [7] this effect was also observed and possible mechanisms for it were discussed. Our experiments thus far performed cannot give a sure explanation for the broadening obtained on the addition of all dyes to the SOL
Fig. 2 shows the Raman spectra of the first three representatives of the homologeous series of 3,3’diet?lyl-2,2’-benzthiacyanines (dyes A, B, C) adsorbed on silver colloids. In ref. [S] these three dyes were investigated in 5 X 10-s -5 X 10-s mol/Q methanolic and methanolic aqueous or aqueous solutions, respectively. In those esperiments in solutions different optimum excitation wavelengths were necessary to get Raman spectra due to the different absorption max-
ima of the dyes. Because of their strong fluorescence and the small photo stability, some dyes could be investigated by CARS only [S] _As shown in Gg_ 2, the same dyes adsorbed on silver colloids gave rise to easily observable Raman spectra in concentration ranges 1O-3 -IO-” times smaller. A further advantage of Raman investigations of polyrnethine
dyes adsorbed on silver colloids in com-
parison with dye solutions is the suppression of fluorescence and a better photo stability. Contrary to the experiments performed in solutions, the different adsorbed dyes of the ~1onloIogeous series could be investigated by the same excitation wavelength (5 13.5 nm, see fig. 3), in spite of their different absorption maxima. This gives hints that the enhancement
Fi& 1. Absorptmn spectra of 0.5 x IO9 mol/P silver sol (1); 1 : 1 mi\ture of 10e3 mol/S silver sol with 1.1 X lo6 mol/P .~queousdye C soturion (1) and 0.7 x IO* mol/P aqueous solurion of dpe C (3). Optical path for (l)-(3) t\as 0.2 cm.
upon adsorption on the metal for the different dyes must be different or that for adsorbed dye molecules the resonance enhancement due to the specific molecular absorption, which was necessary for successful Raman scattering in solutions, is not essential. We did not find for the investigated dyes a zero surface enhancement for dyes as discussed [63 _ A detaiIed discussion of the scattering behaviour of the homologous series (dyes A, B, C) in the light of models describing SERS will be given in a separate publication_ Fig. 3 shows the Raman spectra of merocyanine (dye D) in methanolic solution (a) and adsorbed on silver colloids (b). Also this dye shows an essential growth of Raman scattering intensity after adsorption (see experimental conditions given in fig. 3). The frequency differences between cyanine dyes in solution [8] and adsorbed on silver colloids (*IO cm-l)
CHEMICAiPHYSICS
Volume 99, number 5,6
a
I’
5
19 Au-gist-3983
iETTERS
b m
B
I
1400 f Lfye:
3406 I Oy*:
c
too0 t
606
ZOD
I
mm f
600
201
13
Fii. 2. Raman spectra of dye A {a), dye B fb) and dye C (c) adsorbed on silver sol from lCJ_? molfn aqueous solution at nearly the same esperimental conditions.
Volume 99.
CHEMJCAL PHYSICS LETTERS
number 5,6
19 August 1983
b
lb00 Dye:
1000
600
1400
ZOO
I
Dye:
D
1000
I
4. Conclusions
SERS on silver colloids is shown to be a powerful and simple method for studying vibrational spectra of polymethine dyes. Some methodolo@al problems connected with Raman spectroscopic investigations of these molecules were overcome_ Potential sensitizing dyes especially seem to have rather good adsorption properties and efficient SERS. In this way vibrational spectroscopic investigations on these dyes could be easily performed, resulting in additional information about sensitizing molecules and spectral sensitization. 506
200
D
Fip. 3. Raman spectra of dye D in Z X 10v3 methanolic solution (a) and of 10m7 mol/P aqueoussohrtuon sot (b): Botb spectra v.ere registmted with the same expcrinrental conditions.
are of the sdtne order observed on changes in the solvent 191. As expected for the dipolar merocyanine, the frequency shifts between dissolved and adsorbed molecules are more evident- A discussion of the frequency shiftingand intensity behaviour of the different Raman lines will be given later.
600 t
mixed I : 1 with silver
References [ 11 DJ. Jeanmaire and R.P. van Duyne, J. Electroam& 64 (1977) [ 2]
Chem.
1.
J-A. Creighton, CC. Blatchford and hi.G. Albrecht, J.
Chem. Sot. Faraday Trans. II 75 (1979) 970. [3] A-V. Baranov and Y-S. Bobowitsch. Opt. Spectry. 52 (1982) 385. ]4] A.V_ Baranov and Y-S. Bobowitsch, Soviet Phys. JETP Letters 35 (1982) 149. [S] PC. Lee and D. Meisel, J. Phys. Chem. 86 (1982) 3391. [ 61 A. Bachackashvilh. S. Efrima, 8. Katz and 2_ Priel, Chem. Phys. Letters 94 (1983) 571. [ 71 A.V. Baranov and Y.S. Bobovitsch, Soviet Phys. JETP Letters 36 (1982) 277. IS] A- Lau, K_ Kneipp, W_ Werncke, K_ Lenz, HJ_ Weigmann, D. Fassler and G. Hinzmann, Advan. Mol. Relaxation Interaction Processes 24 (19S2) 27. [9] G. Hinzmarm K. Kneipp and D. Fassier, to be published.