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Filters coated with colloidal silver for SER spectroscopy
Journal of
ELSEVIER
MOLECULAR STRUCTURE Journal of Molecular Structure 348 (1995) 277-280
Filters coated with Colloidal Silver for SER Spectroscopy Natale Neto a, Maurizio Muniz-Miranda a and Giuseppe Sbrana b a Dipartimento di Chimica, Universita’ di Firenze, Via G. Capponi 9, I-50121 Firenze, Italy b Centro CNR Composti Eterociclici, Via G. Capponi 9, I-50121 Firenze, Italy
2,2’-bipyridine and 2-amino,!5nitropyridine were adsorbed on stable SERS-active substrates, prepared by filtration of silver colloidal dispersions through alluminum oxide membranes. SERS spectra were thus obtained and compared with those of corresponding experiments on colloidal dispersions.
1.INTRODUCTION Silver colloidal particles dispersed as hydrosols represent a very convenient substrate for obtaining SERS spectra of organic molecules despite the fact that a colloidal dispersion is a very unstable system, undergoing aggregation or coagulation phenomena which affect the adsorption process and the reproducibility of the spectral data [l]. In addition, the presence of water molecules and ions perturbs the equilibrium of the electrostatic double layer at the metal surface and produces competitive adsorption processes which are often difficult to analyze. Most of these difficulties can be eliminated by using SERS-active silver surfaces formed by filters coated with colloidal silver, obtained by filtration of colloidal dispersions through alluminum oxide membranes. In this way the filter surface is covered by a layer of silver particles on which organic molecules, including those insoluble in aqueous solutions, can be adsorbed and then studied by SER spectroscopy. This simple procedure produces a layer of colloidal particles similar, in size, to those present in the aqueous solutions, thus Raman scattering intensities of adsorbates on coated filters are comparable with those obtained from colloidal dispersions. Coated filters may be washed to remove adsorbed ions in order to identify the effect of w-adsorbed ions on the SER spectra of organic molecules. Silver coated paper filters, obtained by coagulation of silver hydrosols, were also proposed [2] as suitable substrates for SERS experiments. In this case, however, the addition of excess electrolite, necessary for sol coagulation, modifies the ionic strength of the solution and the size of the colloidal particles. 0022-2860/95/$09.50 0 1995 Elsevier Science B.V. SSDI 0022-2860(95)08642-O
All rights reserved
278
P.EXPERIMENTAL Silver colloidal dispersions, prepared following the method of Ref. [3], were filtered through ANOTOP filters (Anotec, U.K.) with 0.02 pm filtration diameter. After two successive filtrations, the liquid is practically free of dispersed particles and the filter is coated by a thin layer of colloidal silver, forming a SERS-active surface. To remove adsorbed ions deriving from the solution, this substrate was washed with three-distilled water and with pure ethanol, in an ultrasonic bath, and then dried at 70-80 OC. The washing step was not carried out for experiments in presence of coadsorbed ions originating from the solution. Adsorption of organic molecules on silver coated filters was achieved by wetting the substrate with an appropriate diluted solution. SERS measurements were carried out by illuminating the filter surface with the 514.5 nm beam of a Ar+ laser, at an incidence angle of 450. To avoid local heating, the laser beam (20 mW) was defocalized and the filters were mounted on a rotating device.
3.RESULTS Filters coated with colloidal silver form a stable metal substrate for SERS experiments of adsorbed organic molecules, leading to Raman spectra almost identical to those obtained from colloidal dispersions but free of unwanted contribution of coadsorbed ions like, for instance, borate ions formed when silver is reduced following the Ref. [3]. A comparison between results obtained with the two different substrates is presented in what follows, for the specific case of two molecules represented by 2,2’-bipyridine and 2-amino+nitropyridine.
SERS of 2,2’-bipyridine In a SERS investigation [4] of 2,2’-bipyridine adsorbed on Ag colloids two different adsorbed species, I and II, were identified, depending on the pH of the colloidal dispersion and on the presence of halide anions. According to Ref. [4], at the normal pH=9.5 value of the colloidal dispersion, species I is invariably observed. This result does not change by addition of KCI or KBr while 2,2’-bipyridine adsorbs as species II in presence of I- anions. Species II is also observed at much lower pH values but only in presence of Cl- and Br. Species I is associated to an adsorption of the molecule in a cis planar conformation, similar to that assumed in the argentous coordination compound. The SER spectrum of species I is reported in Fig. IA and it is characterized by the occurrence of a ring breathing vibration at about 1010 cm-l. The two strong bands at 612 and 924 cm-1 are attributed to vibrations of coadsorbed borate ion [5] and disappear when Br or Cl- are added to the colloidal dispersion, see Fig. IB. In the latter spectrum the intense As-Cl stretching vibration at 244 cm-1 covers the Ag-N band observed in the anion-free spectrum of Fig. IA. Comparison of Fig. 1A with Fig. IB confirms that the simple addition of appreciable amount of Cl-, without the presence of hydrogen ions, does not produce adsorption of species II.
279
>-
>
e -
I--
Ln
m z
7
w c
W
z -
z -
+
500
1000
Wavenumbers
15'00
(cm-l)
Fig.1. SERS of 2,2’-bipyridine:(A) salt-free colloid; (B) colloid with lo-*M NaCI.
Wavenumbers
(cm-')
Fig.2. SERS of 2,2’-bipyridine: (A) silver coated filter; (B) silver coated filter washed in ultrasonic bath.
Species II corresponds to a strongly chemisorbed molecule with a Lewis acid coordination structure. As shown in Fig. 28, the ring breathing vibration of species II is shifted to 1023 cm-1 and additional SERS bands, not present for species I, occur at 371, 660, 1320, 1660 and 1602 cm-! This spectrum, obtained by us from silver coated films washed in an ultrasonic bath, turns out to be identical to that reported in Ref. [4] for a colloidal dispersion at pH=2.8 and in presence of Cl- or Br anions. A similar result was obtained without washing the filter, see Fig. 2A, although a careful examination of the latter spectrum shows that a not negligible amount of species I coexists with the type II species. In view of these results, it is possible to conclude that coadsorbed Cl- and Br, together with H+, cause desorption from silver particles of anions, mainly hydroxide, present in the colloidal dispersion and this is a prerequisite for a strongly adsorbed 2,2’-bipyridine as type II species. The same role of anion displacement is played by I-, whose affinity with silver is much greater than that of the other halides.
SERS of 2-amino, 5-nitropytidine 2-Amino$nitropyridine (hereafter ANP) adsorbs on silver colloids and electrode [6,7] as ANP- anion, in presence of hydroxide ions, and as a neutral molecule, at acidic pH. The SER spectra from silver colloids are reported in Fig. 3, for ANP adsorbed as anion (Fig. 3A) and neutral molecule (Fig. 3B). Corresponding spectra when the adsorption is carried out on coated filters are shown in Fig. 4. After filtering the colloidal dispersion (at the usual pH=9.5) and drying the surface without the washing procedure, anions deriving from the solution, mainly OH-, are still present on the substrate. Hence, by wetting the filter with an alcoholic solution of
280 ANP, the SER spectrum of ANP- is observed (Fig. 4A), practically identical to that obtained from the colloidal dispersion. If the filters are instead washed before adding the alcoholic solution, thus removing adsorbed ions deriving from the solution, the SER spectrum of the neutral molecule (Fig. 48) is obtained, identical to that of Fig. 3B originated from colloids in presence of hydrogen ions. This confirms that hydroxide ions, present in the colloidal dispersion and coadsorbed on the silver substrate, are responsible of the formation of ANP- anions at the SERSactive sites.
500
1000
1500
Wavenumbers (cm-l) Fig.3. SERS of ANP: (A) silver colloid at pH=9.5; (B) silver colloid at pH=5.
500
1000
1500
Wavenumbers (cm-l) Fig.4. SERS of ANP: (A) silver coated filter; (B) silver coated filter washed in ultrasonic bath.
REFERENCES 1. J.C. Cook, C.M.P. Cuypers, B.J. Kip, R.J. Meier and E. Koglin, J. Raman Spectrosc. 24 (1993) 609. 2. P. Hildebrandt, S. Keller, A. Hoffmann, F. Vanhecke and B. Schrader, J. Raman Spectrosc. 24 (1993) 791. 3. J.A. Creighton, C.G. Blatchford and M.G. Albrecht, J. Chem. Sot. Faraday Trans 2, 75 (1979) 790. 4. M. Kim and K. Itoh, J. Phys. Chem. 91 (1987) 126. 5. G. Sbrana, N. Neto, M. Muniz-Miranda and M. Nocentini, J. Phys. Chem. 94 (1990) 3706. 6. C.C. Busby and J.A. Creighton, J. Electroanal. Chem. 133 (1982) 183. 7. M.L. Foresti, A.M. Funtikov, R. Guidelli and M. Muniz-Miranda, J. Electroanal. Chem. 367 (1994) 223.
ELSEVIER
MOLECULAR STRUCTURE Journal of Molecular Structure 348 (1995) 277-280
Filters coated with Colloidal Silver for SER Spectroscopy Natale Neto a, Maurizio Muniz-Miranda a and Giuseppe Sbrana b a Dipartimento di Chimica, Universita’ di Firenze, Via G. Capponi 9, I-50121 Firenze, Italy b Centro CNR Composti Eterociclici, Via G. Capponi 9, I-50121 Firenze, Italy
2,2’-bipyridine and 2-amino,!5nitropyridine were adsorbed on stable SERS-active substrates, prepared by filtration of silver colloidal dispersions through alluminum oxide membranes. SERS spectra were thus obtained and compared with those of corresponding experiments on colloidal dispersions.
1.INTRODUCTION Silver colloidal particles dispersed as hydrosols represent a very convenient substrate for obtaining SERS spectra of organic molecules despite the fact that a colloidal dispersion is a very unstable system, undergoing aggregation or coagulation phenomena which affect the adsorption process and the reproducibility of the spectral data [l]. In addition, the presence of water molecules and ions perturbs the equilibrium of the electrostatic double layer at the metal surface and produces competitive adsorption processes which are often difficult to analyze. Most of these difficulties can be eliminated by using SERS-active silver surfaces formed by filters coated with colloidal silver, obtained by filtration of colloidal dispersions through alluminum oxide membranes. In this way the filter surface is covered by a layer of silver particles on which organic molecules, including those insoluble in aqueous solutions, can be adsorbed and then studied by SER spectroscopy. This simple procedure produces a layer of colloidal particles similar, in size, to those present in the aqueous solutions, thus Raman scattering intensities of adsorbates on coated filters are comparable with those obtained from colloidal dispersions. Coated filters may be washed to remove adsorbed ions in order to identify the effect of w-adsorbed ions on the SER spectra of organic molecules. Silver coated paper filters, obtained by coagulation of silver hydrosols, were also proposed [2] as suitable substrates for SERS experiments. In this case, however, the addition of excess electrolite, necessary for sol coagulation, modifies the ionic strength of the solution and the size of the colloidal particles. 0022-2860/95/$09.50 0 1995 Elsevier Science B.V. SSDI 0022-2860(95)08642-O
All rights reserved
278
P.EXPERIMENTAL Silver colloidal dispersions, prepared following the method of Ref. [3], were filtered through ANOTOP filters (Anotec, U.K.) with 0.02 pm filtration diameter. After two successive filtrations, the liquid is practically free of dispersed particles and the filter is coated by a thin layer of colloidal silver, forming a SERS-active surface. To remove adsorbed ions deriving from the solution, this substrate was washed with three-distilled water and with pure ethanol, in an ultrasonic bath, and then dried at 70-80 OC. The washing step was not carried out for experiments in presence of coadsorbed ions originating from the solution. Adsorption of organic molecules on silver coated filters was achieved by wetting the substrate with an appropriate diluted solution. SERS measurements were carried out by illuminating the filter surface with the 514.5 nm beam of a Ar+ laser, at an incidence angle of 450. To avoid local heating, the laser beam (20 mW) was defocalized and the filters were mounted on a rotating device.
3.RESULTS Filters coated with colloidal silver form a stable metal substrate for SERS experiments of adsorbed organic molecules, leading to Raman spectra almost identical to those obtained from colloidal dispersions but free of unwanted contribution of coadsorbed ions like, for instance, borate ions formed when silver is reduced following the Ref. [3]. A comparison between results obtained with the two different substrates is presented in what follows, for the specific case of two molecules represented by 2,2’-bipyridine and 2-amino+nitropyridine.
SERS of 2,2’-bipyridine In a SERS investigation [4] of 2,2’-bipyridine adsorbed on Ag colloids two different adsorbed species, I and II, were identified, depending on the pH of the colloidal dispersion and on the presence of halide anions. According to Ref. [4], at the normal pH=9.5 value of the colloidal dispersion, species I is invariably observed. This result does not change by addition of KCI or KBr while 2,2’-bipyridine adsorbs as species II in presence of I- anions. Species II is also observed at much lower pH values but only in presence of Cl- and Br. Species I is associated to an adsorption of the molecule in a cis planar conformation, similar to that assumed in the argentous coordination compound. The SER spectrum of species I is reported in Fig. IA and it is characterized by the occurrence of a ring breathing vibration at about 1010 cm-l. The two strong bands at 612 and 924 cm-1 are attributed to vibrations of coadsorbed borate ion [5] and disappear when Br or Cl- are added to the colloidal dispersion, see Fig. IB. In the latter spectrum the intense As-Cl stretching vibration at 244 cm-1 covers the Ag-N band observed in the anion-free spectrum of Fig. IA. Comparison of Fig. 1A with Fig. IB confirms that the simple addition of appreciable amount of Cl-, without the presence of hydrogen ions, does not produce adsorption of species II.
279
>-
>
e -
I--
Ln
m z
7
w c
W
z -
z -
+
500
1000
Wavenumbers
15'00
(cm-l)
Fig.1. SERS of 2,2’-bipyridine:(A) salt-free colloid; (B) colloid with lo-*M NaCI.
Wavenumbers
(cm-')
Fig.2. SERS of 2,2’-bipyridine: (A) silver coated filter; (B) silver coated filter washed in ultrasonic bath.
Species II corresponds to a strongly chemisorbed molecule with a Lewis acid coordination structure. As shown in Fig. 28, the ring breathing vibration of species II is shifted to 1023 cm-1 and additional SERS bands, not present for species I, occur at 371, 660, 1320, 1660 and 1602 cm-! This spectrum, obtained by us from silver coated films washed in an ultrasonic bath, turns out to be identical to that reported in Ref. [4] for a colloidal dispersion at pH=2.8 and in presence of Cl- or Br anions. A similar result was obtained without washing the filter, see Fig. 2A, although a careful examination of the latter spectrum shows that a not negligible amount of species I coexists with the type II species. In view of these results, it is possible to conclude that coadsorbed Cl- and Br, together with H+, cause desorption from silver particles of anions, mainly hydroxide, present in the colloidal dispersion and this is a prerequisite for a strongly adsorbed 2,2’-bipyridine as type II species. The same role of anion displacement is played by I-, whose affinity with silver is much greater than that of the other halides.
SERS of 2-amino, 5-nitropytidine 2-Amino$nitropyridine (hereafter ANP) adsorbs on silver colloids and electrode [6,7] as ANP- anion, in presence of hydroxide ions, and as a neutral molecule, at acidic pH. The SER spectra from silver colloids are reported in Fig. 3, for ANP adsorbed as anion (Fig. 3A) and neutral molecule (Fig. 3B). Corresponding spectra when the adsorption is carried out on coated filters are shown in Fig. 4. After filtering the colloidal dispersion (at the usual pH=9.5) and drying the surface without the washing procedure, anions deriving from the solution, mainly OH-, are still present on the substrate. Hence, by wetting the filter with an alcoholic solution of
280 ANP, the SER spectrum of ANP- is observed (Fig. 4A), practically identical to that obtained from the colloidal dispersion. If the filters are instead washed before adding the alcoholic solution, thus removing adsorbed ions deriving from the solution, the SER spectrum of the neutral molecule (Fig. 48) is obtained, identical to that of Fig. 3B originated from colloids in presence of hydrogen ions. This confirms that hydroxide ions, present in the colloidal dispersion and coadsorbed on the silver substrate, are responsible of the formation of ANP- anions at the SERSactive sites.
500
1000
1500
Wavenumbers (cm-l) Fig.3. SERS of ANP: (A) silver colloid at pH=9.5; (B) silver colloid at pH=5.
500
1000
1500
Wavenumbers (cm-l) Fig.4. SERS of ANP: (A) silver coated filter; (B) silver coated filter washed in ultrasonic bath.
REFERENCES 1. J.C. Cook, C.M.P. Cuypers, B.J. Kip, R.J. Meier and E. Koglin, J. Raman Spectrosc. 24 (1993) 609. 2. P. Hildebrandt, S. Keller, A. Hoffmann, F. Vanhecke and B. Schrader, J. Raman Spectrosc. 24 (1993) 791. 3. J.A. Creighton, C.G. Blatchford and M.G. Albrecht, J. Chem. Sot. Faraday Trans 2, 75 (1979) 790. 4. M. Kim and K. Itoh, J. Phys. Chem. 91 (1987) 126. 5. G. Sbrana, N. Neto, M. Muniz-Miranda and M. Nocentini, J. Phys. Chem. 94 (1990) 3706. 6. C.C. Busby and J.A. Creighton, J. Electroanal. Chem. 133 (1982) 183. 7. M.L. Foresti, A.M. Funtikov, R. Guidelli and M. Muniz-Miranda, J. Electroanal. Chem. 367 (1994) 223.