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Surface-enhanced raman scattering from amphiphilic and polymer molecules on silver and gold sols
CHEWCAL
VO~UIIIC 95. number Z
SURFACE-ENHANCED ON SILVER
AND
RAMAN
GOLD
SCATTERING
25 February
PHYSICS LElTERS
FROM
AMPHIPHILIC
AND
POLYMER
1983
MOLECULES
SOLS
lized front
1. Ititrodttcrion
rhe
acetone.
This salt was then used to prepare
Br- and I- salts by precipitation of the chloride
salt with NaBr and Nal. respectively_ recrystallized
These
were then
from acetone.
Cetylquinolinium bromide was prepared as follows: 10 g cetyl bromide (Hopkin and Williams) and 5.5 g quinoline (_4jasj were heated together at 60-70°C for 5 h. The red product which solidified on cooling was dissolved in 9 : 1 diossne/acetone_ The recrystallized pink crystals which formed had a melting point range of 104- 106°C. This method was adapted from Shelton et al. [?I_ The polyvinylpyrrolidone (m,,. = 44000)* was purified by repeated precipitation from acetone--water. The silver and gold sols were prepared by reduction of AgNO; and KAuCl~. respectively, using NaBH, as described by Creighton et al_ [3] _followed by heating the sol to boiling point to hydrolyse unused NaBH,. Al aqueous solutions were prepared using triply distilled water.
A Spes Ranmlog specrromerer. with facilities for data acquisition, was used for the Rarnan spectra. Sampies were contained in glass capillary tubes with scattered light collected at 90” to the incident beam.
2. Expcritiietttal
(‘et?‘II,\.ririiliitttn
chloride
( BDll
Ltd.) was recrysral’ The PVP sample was kindly supplied by B. Vincent,
Univzr-
sity of Bristol.
0 009-2614/S3/0000-0000/S
03.00
0 1983 North-Holland
Volume 95, number 2
CHEMICAL PHYSICS LETTERS
A 4 cm-l band-pass and laser powers of 100-200 mW were typically employed_ Spectra of the silver samples, as well as normal solution spectra, were recorded using the 5 14.5 nm argon-ion laser line. Gold samples required the 647.1
nm krypton line_
Addition of the cetylpyridinium and cetylquinolinium salts to the Ag and Au sols resulted in a color change of the sol, similar to that described by Creighton et al. [3] _ In a previous paper [4] we observed that the color change was due to the aggregation of the primary metal particles into clusters with little or no coalescence occurring. Silver sols changed from ykllow (no adsorbate) to green/grey, while gold sols varied from red to blue. It was noted that the rate of the color change (degree of aggregation) was somewhat dependent on the surfactam counter-ion, increasing in the order Cl- < Br- < I-. The SER spectra for the cetylpyridinium, cetyl-
I
quinolinium salts and PVP are in figs. I-3.
Apart from
slight intensity differences and the position of the AgX band in the low-frequency region, the spectra of the various halide salts for both the surfactants were very similar- Due to a lower degree of SER enhancement
3 _Results
lb00
25 February 1983
on
gold, the SER spectrum of this surfactant on the gold sol only revealed the dominant ring breathing band near 1030 cm-‘. The solution spectrum at lo5 times the concentration was similarly weak (fig_ 1). We were unable to obtain a normal solution spectrum of the cetylquinolinium salts due to their low scattering profile. When PVP was added to either a red Au sol or a yellow Ag sol, the visible appearance of the sol remained unchanged and no SERS was observed_ By adding small amounts of Mg(CIO& * (-2 X 1Oa hl) to the primary sol, which induced a color change, and then adding PVP (note this prevents further color change, since PVP acts * Addition of NaC104 caused the same effect, but appreciably higher concentrations of this salt were required.
.I
1
YW
1200
1003
800
600
LOCI
WAVENUMBER 1cm-r I
Fig. 1. (a) Raman solution spectrum of cetylpyridinium bromide (0.1 hl aqueous). (b) SER spectrum of cetylpyridinium bromide (8 x lo-’ M) on a Ag sol. The same instrumental settings were used for both (180-200 mW, h = 514.5 nm. P-C. = lo3 cts/;&c).
volilnlc
25 February* 19S3
CHEMICAL PHYSICS LE-ITERS
95, n11r11ber z
WAVENUMBER (cm-r) Fig. 7. SEKS ‘,isct)-lquinoliurn
brunlide (10 -6 >I) on u Af sol (I SO-140
SI_‘IG is observed_ If salt is added. up to 1O-2 31. after PVP has been added to the primary sol. no color chnges or SERS is observed. Similar observations were found if the sol is aggregatcd. prior to I’VPaddition. by aging with no ClO$
3s ;L stabilizer).
mW. A = 5 14.5 nm,
P.C. = 104
cts/scale).
present. This removes any ClOi contributions to the spectrum but the extent of aggregation is less controlled_ Fig. 3~ shows a PVP/Au sol prepared in this way. Since the ClOi band near 930 cm-* coincides with the ring-breathing band in PVP, this band in fig. 3b may
‘WWENUMEiER 1 cm-11 l:ig. 3. (3) IWl’ (*S’;L)
Au WI (vi&r -blue. 156
in ;~queour solution. (b) PVP (0.2%) ;Ig:cd - no (30;
presr’nt) (180-200
on a Ag sol (green) with addes Cl05
mW. P.C. = 3 X lo3
cts/scale. (a).
(2 X lo4
(b). h = 514.5
hi). (C) PVP (0.1%) on a nm. (c) A = 647.1 nm).
Volume 95, number 2
-25 February 1983
CHEMICAL PHYSICS LE-ITERS
arise from either or both species_ Alternatively;
the
PVP ring band may have shifted to higher energy on adsorption, giving the strong band near 1000 cm-’ _ The cluster of bands near 1420 cm-’ in the solution spectrum narrows for the polymer on Ag, and is not
relevant that scanning electron
micrographic
esamina-
tion of the surface of silver electrodes
4. Discussion
exhibiting SERS shows a very elaborate microstructure [S] _Theoretical models for the SERS effect, based on enhanced electric fields around an isolated metal particle, have been produced by several authors [9] _The present work with aggregates suggests the importance of modelling electric field perturbations by a collection of particles in electrical contact, as sugested by hloskovits [lo] _ At-
In previous work [4] we found that the average diameter of particles in both the Ag and Au sols used here was lo-20 nm. These particles cluster in what ap-
tempts in this area are still scarce_ They include two spheres in electrical contact [ 111, a plane and sphere system [la], and a randomly distributed collection of hemispheroids [ 131.
seen on the Au substrate.
pears to be a loose network, and this aggregate is responsible for the observed color changes of the sol. Since there is virtually no coalescence of the primary particles, the total surface area of the sol does not change significantly when aggregates are fomled. Hence, we can calculate the surface of the sol based on the primary particle data. Typically, for the given preparation ([Ag] = lo-; M), the surface area of the sol is about 1.2 m2jP. The surfactant molecules used have a packing area of GO-100 A’ per molecule [5] : thus at surfactant concentrations of 10e6 M, less than full surface coverage would occur even if all surfactant molecules were adsorbed. This affirms that while the mechanism responsible for SERS may persist at distances beyond monolayer thickness, additional layers are not required. Similar conclusions have been drawn from studies concerned with vapour adsorbed on metal films [6] _ The behaviour noted for the PVP systems tempts us to speculate on the influence of aggregation on SERS. The observation that PVP stabilizes both the yellow and the green Ag sols indicates that the PVP is adsorbed onto both types of sol_ However, only the green sol, i.e. the one with the aggregated structure, shows the SERS effect_ This is a strong indication that the aggregated structure is essential for SERS. Some
support
for the view that a loose aggregate of small
metal spheres have special surface properties, as compared to isolated spheres, is provided by the recent work of Batson [7]. In that case several aluminium spheres in contact showed estra surface plasmon states as compared to the isolated spheres_ Although the sur-
face plasmon
states were detected
by an electron
scat-
tering method (EELS) rather than by optical techniques, this should not be significant_ It may also be
5. Conclusions The present study shows that SERS can be observed from amphiphilic molecules at sub-monolayer coverages of colloidal metal surfaces. It has also been found that aggregation of the colloidal substrate is an important factor for observing enhanced Raman scatter from an adsorbed molecule_ Whether this aggregation introduces new surface plasmon states to the system or only acts in perturbing the electric field of the light still remains to be answered.
Acknowledgement
We thank Dr. Brian Vincent for suggesting the use of PVP as a possible candidate for e_xhibiting SERS. ShlH acknowledges support from a Commonwealth Postgraduate
Research
Award.
References [ 1] J. Bontous, A. Dtluphn and R. Xui~nan. J. Ctrim. Plryr
66 (1969) 1259: B. Jirgensons. !&&ron~ol. Chem_ 6 (195 1) 30_ 171 R.S. Shelton. b1.G. van Campen, C.H. Tilford, H.C. Lzng. L. Nisonzer, FJ. Bandelin and H.L. Rubenkoenig. J. Am. Chem. Sot. 68 (1946) 757. [3] J-A. Crei_ghton.CC. Blntchford and 31.G. AlbrecIit. Trans. Faraday Sot. 75 (1979) 790. 141 S.M. Heard, F. Gricser, CC. Barrxlough and J-V. Sanders. J. Colloid ht. Sci.. to be published. [S] P. hfukerjee and A. Anavil, in: Adsorption at Interfaces. Am. Chem. Sot. Symp_ Ser. S rd. E.L. blirtal(l975). 157
\‘olumc
95, number
2
CHEMICAL
161 D-P. DiLclla. A. Gohin. R.H. Lipson, P. McBrecn and hl. Moslwvirs. 1. Chcm. Phys. 73 (1980) 4282; 11. S&i and M.R. Philpotr. J. Chcm. Phys. 73 (1980) 5376: T.11. Wood. ht.\‘. Klein and D.A. Zwcmcr. Surfice Sci. 107 (1961) 625. I\‘. KIWII. h1.R. Philpott and W.G. Golden. J. Chem. Phys. 77 (1981) 119. 171 P.H. Batsun. Phys. Rev. Lcttcrs 49 (1982) 936. 1t3] 511. .\Ixo~nbcr. TX. Furtak ;Ind T-M. Devinc, Chem. I’hyk Lctrcrs 90 (1982) 439.
PHYSICS
LETTERS
25 February
1983
[S] J. Gersten and A. Nitzan. J. Chem. Phys. 73 (1980) 3023; hl. Kerker, D--S. Wmg and H. Chew, Appl. Optics 19 (1980) 4159; F-J. Adrian, Chem. Phys. Letters 78 (1981) 45. 1 IO] hi. hfoskovits, J. Chem. Phys. 69 (1978) 4159. [ 1 l] P.K. Aravind, A. Nitzan and H. Metiu, Surface Sci. 110 (1981) 189. [ 171 P-K;. Aravind. R.W. Rendell and H. Metiu, Chem. Phys. Letters 85 (1982) 396. 113) U. Laor and G-C. Schatz. Chem. Phys. Letters 82 (1981) 566; J. Chem. Phys. 76 (1982) 2888.
VO~UIIIC 95. number Z
SURFACE-ENHANCED ON SILVER
AND
RAMAN
GOLD
SCATTERING
25 February
PHYSICS LElTERS
FROM
AMPHIPHILIC
AND
POLYMER
1983
MOLECULES
SOLS
lized front
1. Ititrodttcrion
rhe
acetone.
This salt was then used to prepare
Br- and I- salts by precipitation of the chloride
salt with NaBr and Nal. respectively_ recrystallized
These
were then
from acetone.
Cetylquinolinium bromide was prepared as follows: 10 g cetyl bromide (Hopkin and Williams) and 5.5 g quinoline (_4jasj were heated together at 60-70°C for 5 h. The red product which solidified on cooling was dissolved in 9 : 1 diossne/acetone_ The recrystallized pink crystals which formed had a melting point range of 104- 106°C. This method was adapted from Shelton et al. [?I_ The polyvinylpyrrolidone (m,,. = 44000)* was purified by repeated precipitation from acetone--water. The silver and gold sols were prepared by reduction of AgNO; and KAuCl~. respectively, using NaBH, as described by Creighton et al_ [3] _followed by heating the sol to boiling point to hydrolyse unused NaBH,. Al aqueous solutions were prepared using triply distilled water.
A Spes Ranmlog specrromerer. with facilities for data acquisition, was used for the Rarnan spectra. Sampies were contained in glass capillary tubes with scattered light collected at 90” to the incident beam.
2. Expcritiietttal
(‘et?‘II,\.ririiliitttn
chloride
( BDll
Ltd.) was recrysral’ The PVP sample was kindly supplied by B. Vincent,
Univzr-
sity of Bristol.
0 009-2614/S3/0000-0000/S
03.00
0 1983 North-Holland
Volume 95, number 2
CHEMICAL PHYSICS LETTERS
A 4 cm-l band-pass and laser powers of 100-200 mW were typically employed_ Spectra of the silver samples, as well as normal solution spectra, were recorded using the 5 14.5 nm argon-ion laser line. Gold samples required the 647.1
nm krypton line_
Addition of the cetylpyridinium and cetylquinolinium salts to the Ag and Au sols resulted in a color change of the sol, similar to that described by Creighton et al. [3] _ In a previous paper [4] we observed that the color change was due to the aggregation of the primary metal particles into clusters with little or no coalescence occurring. Silver sols changed from ykllow (no adsorbate) to green/grey, while gold sols varied from red to blue. It was noted that the rate of the color change (degree of aggregation) was somewhat dependent on the surfactam counter-ion, increasing in the order Cl- < Br- < I-. The SER spectra for the cetylpyridinium, cetyl-
I
quinolinium salts and PVP are in figs. I-3.
Apart from
slight intensity differences and the position of the AgX band in the low-frequency region, the spectra of the various halide salts for both the surfactants were very similar- Due to a lower degree of SER enhancement
3 _Results
lb00
25 February 1983
on
gold, the SER spectrum of this surfactant on the gold sol only revealed the dominant ring breathing band near 1030 cm-‘. The solution spectrum at lo5 times the concentration was similarly weak (fig_ 1). We were unable to obtain a normal solution spectrum of the cetylquinolinium salts due to their low scattering profile. When PVP was added to either a red Au sol or a yellow Ag sol, the visible appearance of the sol remained unchanged and no SERS was observed_ By adding small amounts of Mg(CIO& * (-2 X 1Oa hl) to the primary sol, which induced a color change, and then adding PVP (note this prevents further color change, since PVP acts * Addition of NaC104 caused the same effect, but appreciably higher concentrations of this salt were required.
.I
1
YW
1200
1003
800
600
LOCI
WAVENUMBER 1cm-r I
Fig. 1. (a) Raman solution spectrum of cetylpyridinium bromide (0.1 hl aqueous). (b) SER spectrum of cetylpyridinium bromide (8 x lo-’ M) on a Ag sol. The same instrumental settings were used for both (180-200 mW, h = 514.5 nm. P-C. = lo3 cts/;&c).
volilnlc
25 February* 19S3
CHEMICAL PHYSICS LE-ITERS
95, n11r11ber z
WAVENUMBER (cm-r) Fig. 7. SEKS ‘,isct)-lquinoliurn
brunlide (10 -6 >I) on u Af sol (I SO-140
SI_‘IG is observed_ If salt is added. up to 1O-2 31. after PVP has been added to the primary sol. no color chnges or SERS is observed. Similar observations were found if the sol is aggregatcd. prior to I’VPaddition. by aging with no ClO$
3s ;L stabilizer).
mW. A = 5 14.5 nm,
P.C. = 104
cts/scale).
present. This removes any ClOi contributions to the spectrum but the extent of aggregation is less controlled_ Fig. 3~ shows a PVP/Au sol prepared in this way. Since the ClOi band near 930 cm-* coincides with the ring-breathing band in PVP, this band in fig. 3b may
‘WWENUMEiER 1 cm-11 l:ig. 3. (3) IWl’ (*S’;L)
Au WI (vi&r -blue. 156
in ;~queour solution. (b) PVP (0.2%) ;Ig:cd - no (30;
presr’nt) (180-200
on a Ag sol (green) with addes Cl05
mW. P.C. = 3 X lo3
cts/scale. (a).
(2 X lo4
(b). h = 514.5
hi). (C) PVP (0.1%) on a nm. (c) A = 647.1 nm).
Volume 95, number 2
-25 February 1983
CHEMICAL PHYSICS LE-ITERS
arise from either or both species_ Alternatively;
the
PVP ring band may have shifted to higher energy on adsorption, giving the strong band near 1000 cm-’ _ The cluster of bands near 1420 cm-’ in the solution spectrum narrows for the polymer on Ag, and is not
relevant that scanning electron
micrographic
esamina-
tion of the surface of silver electrodes
4. Discussion
exhibiting SERS shows a very elaborate microstructure [S] _Theoretical models for the SERS effect, based on enhanced electric fields around an isolated metal particle, have been produced by several authors [9] _The present work with aggregates suggests the importance of modelling electric field perturbations by a collection of particles in electrical contact, as sugested by hloskovits [lo] _ At-
In previous work [4] we found that the average diameter of particles in both the Ag and Au sols used here was lo-20 nm. These particles cluster in what ap-
tempts in this area are still scarce_ They include two spheres in electrical contact [ 111, a plane and sphere system [la], and a randomly distributed collection of hemispheroids [ 131.
seen on the Au substrate.
pears to be a loose network, and this aggregate is responsible for the observed color changes of the sol. Since there is virtually no coalescence of the primary particles, the total surface area of the sol does not change significantly when aggregates are fomled. Hence, we can calculate the surface of the sol based on the primary particle data. Typically, for the given preparation ([Ag] = lo-; M), the surface area of the sol is about 1.2 m2jP. The surfactant molecules used have a packing area of GO-100 A’ per molecule [5] : thus at surfactant concentrations of 10e6 M, less than full surface coverage would occur even if all surfactant molecules were adsorbed. This affirms that while the mechanism responsible for SERS may persist at distances beyond monolayer thickness, additional layers are not required. Similar conclusions have been drawn from studies concerned with vapour adsorbed on metal films [6] _ The behaviour noted for the PVP systems tempts us to speculate on the influence of aggregation on SERS. The observation that PVP stabilizes both the yellow and the green Ag sols indicates that the PVP is adsorbed onto both types of sol_ However, only the green sol, i.e. the one with the aggregated structure, shows the SERS effect_ This is a strong indication that the aggregated structure is essential for SERS. Some
support
for the view that a loose aggregate of small
metal spheres have special surface properties, as compared to isolated spheres, is provided by the recent work of Batson [7]. In that case several aluminium spheres in contact showed estra surface plasmon states as compared to the isolated spheres_ Although the sur-
face plasmon
states were detected
by an electron
scat-
tering method (EELS) rather than by optical techniques, this should not be significant_ It may also be
5. Conclusions The present study shows that SERS can be observed from amphiphilic molecules at sub-monolayer coverages of colloidal metal surfaces. It has also been found that aggregation of the colloidal substrate is an important factor for observing enhanced Raman scatter from an adsorbed molecule_ Whether this aggregation introduces new surface plasmon states to the system or only acts in perturbing the electric field of the light still remains to be answered.
Acknowledgement
We thank Dr. Brian Vincent for suggesting the use of PVP as a possible candidate for e_xhibiting SERS. ShlH acknowledges support from a Commonwealth Postgraduate
Research
Award.
References [ 1] J. Bontous, A. Dtluphn and R. Xui~nan. J. Ctrim. Plryr
66 (1969) 1259: B. Jirgensons. !&&ron~ol. Chem_ 6 (195 1) 30_ 171 R.S. Shelton. b1.G. van Campen, C.H. Tilford, H.C. Lzng. L. Nisonzer, FJ. Bandelin and H.L. Rubenkoenig. J. Am. Chem. Sot. 68 (1946) 757. [3] J-A. Crei_ghton.CC. Blntchford and 31.G. AlbrecIit. Trans. Faraday Sot. 75 (1979) 790. 141 S.M. Heard, F. Gricser, CC. Barrxlough and J-V. Sanders. J. Colloid ht. Sci.. to be published. [S] P. hfukerjee and A. Anavil, in: Adsorption at Interfaces. Am. Chem. Sot. Symp_ Ser. S rd. E.L. blirtal(l975). 157
\‘olumc
95, number
2
CHEMICAL
161 D-P. DiLclla. A. Gohin. R.H. Lipson, P. McBrecn and hl. Moslwvirs. 1. Chcm. Phys. 73 (1980) 4282; 11. S&i and M.R. Philpotr. J. Chcm. Phys. 73 (1980) 5376: T.11. Wood. ht.\‘. Klein and D.A. Zwcmcr. Surfice Sci. 107 (1961) 625. I\‘. KIWII. h1.R. Philpott and W.G. Golden. J. Chem. Phys. 77 (1981) 119. 171 P.H. Batsun. Phys. Rev. Lcttcrs 49 (1982) 936. 1t3] 511. .\Ixo~nbcr. TX. Furtak ;Ind T-M. Devinc, Chem. I’hyk Lctrcrs 90 (1982) 439.
PHYSICS
LETTERS
25 February
1983
[S] J. Gersten and A. Nitzan. J. Chem. Phys. 73 (1980) 3023; hl. Kerker, D--S. Wmg and H. Chew, Appl. Optics 19 (1980) 4159; F-J. Adrian, Chem. Phys. Letters 78 (1981) 45. 1 IO] hi. hfoskovits, J. Chem. Phys. 69 (1978) 4159. [ 1 l] P.K. Aravind, A. Nitzan and H. Metiu, Surface Sci. 110 (1981) 189. [ 171 P-K;. Aravind. R.W. Rendell and H. Metiu, Chem. Phys. Letters 85 (1982) 396. 113) U. Laor and G-C. Schatz. Chem. Phys. Letters 82 (1981) 566; J. Chem. Phys. 76 (1982) 2888.