- Email: [email protected]
Influence of Ag colloid aggregation on porphyrin SERRS spectra monitored via Raman correlation spectroscopy
Journal of Molecular Structure 565±566 (2001) 129±132
www.elsevier.nl/locate/molstruc
In¯uence of Ag colloid aggregation on porphyrin SERRS spectra monitored via Raman correlation spectroscopy P. Praus*, M. ProchaÂzka, J. SÏteÏpaÂnek, J. Bok Institute of Physics, Charles University, Ke Karlovu 5, CZ-121 16 Prague 2, Czech Republic Received 31 August 2000; accepted 29 September 2000
Abstract Correlation spectroscopy principle was applied to study Raman signal of the SERS active system consisting of Ag colloid and copper(II) 5,10,15,20-tetrakis (1-methyl-4-pyridyl) porphyrin. A multichannel Raman spectrometer was used to record sets of consecutive Raman spectra with sampling times of tenths or units of seconds. SERRS spectra of two SERS-active formations assigned to the porphyrin molecules on the surface of Ag particles and in the aggregates were isolated by means of the statistical noise analysis. It has been proved, that Raman correlation spectroscopy can be employed to resolve complete Raman spectra coming from either small or large objects levitating in liquid. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Raman correlation spectroscopy; SERRS; Factor analysis
1. Introduction The technique of correlation spectroscopy enables an optical signal from sources the number of which ¯uctuates randomly inside the inspected volume to be distinguished by means of a statistical noise analysis. This concept has been initially introduced in absorption and ¯uorescence [1] spectroscopy. The ¯uctuations of particles in the excited volume can, however, also cause a substantial increase of the Raman signal noise [2]. The ®rst application of Raman correlation spectroscopy, when colloidal suspensions of particles with carotene were studied, was presented two years ago [3]. Two optical ®bres placed in the back focal plane of the spectrometer fed Raman signal from two spectral bands to a couple of photomultipliers, and the correlation of the output signals was monitored. Due to a parallel detection, a CCD system enables * Corresponding author. E-mail address: [email protected] (P. Praus).
to record entire wide spectral regions using sampling times of tenths or units of seconds. These times correspond well to the typical correlation times of solid particles levitating in the excited volume of nonstirred aqueous samples [2]. It indicates a prospect of separating the solid-particle Raman spectra from those of the ªsolutionº. Our contribution concerns an application of this approach to resolve the SERRS spectra of porphyrin molecules adsorbed either on single Ag colloidal particles and or on Ag particles aggregates. 2. Experimental SERS-active system was prepared by addition of copper(II) 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (CuTMPyP) to borohydride-reduced Ag colloid. Final concentration of CuTMPyP in the system was ,2 £ 10 27 M, ensuring only slight colloid aggregation [4].
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(00)00782-1
130
P. Praus et al. / Journal of Molecular Structure 565±566 (2001) 129±132
Fig. 1. (A) Single SERRS spectrum of Ag colloid/CuTMPyP system. (B) Isolated spectrum of CuTMPyP on single colloidal particles (the ¯uctuations of the SERRS signal are not correlated with those of the plasma lines). (C) Spectrum of the porphyrins in aggregates (intensity ¯uctuations correlated with those of the plasma lines). Plasma lines are marked by asterisks.
SERRS spectra were obtained by using a multichannel Raman spectrometer equipped with a Monospec-600 monochromator (60 cm focal length, 1200 grooves/mm grating) and a liquid-N2 cooled CCD detector system (Princeton Instruments) based on an EEV 1024 £ 256 detection element. Spectra were excited by the 457.9 nm Ar 1 laser line. Average laser power at the sample in 1 ml standard Raman cell was about 25 mW. The SERRS signal was collected in a right angle scattering geometry. No bandpass ®lter was used to eliminate Ar 1 plasma lines. One thousand frames of SERRS spectra were recorded
consecutively with 0.5 s exposition time. This exposition time should correspond to the ¯uctuations correlation interval of the signal from SERRS-active inhomogeneous particles. Comparable correlation interval has been observed previously on a similar inhomogeneous system [2]. 3. Statistical analysis of SERRS spectra The spectra contained both the porphyrin SERRS bands and the plasma lines (Fig. 1). Each spectrum
P. Praus et al. / Journal of Molecular Structure 565±566 (2001) 129±132
was ®rst divided into segments covering exclusively or the SERRS bands or the plasma lines (the spectral regions with overlap of both spectral features were omitted), and the two sets of spectral fragments were then separately treated by a factor analysis (FA) [5]. FA (`singular value decomposition' algorithm) provides set of orthonormal subspectra S, matrix of orthonormal coef®cients V and vector of singular numbers W. Particular experimental spectrum Yi n can then be approximated as
Yi n
M X j1
Vij Wj Sj n
1
The factor dimension M represents the number of independent relevant components resolvable in the analysed spectral set. Its value can be derived from the decrease of the signal-to-noise ratio, diminution of the singular number values, or subsidence of the residual error [5] with increasing j. The factor dimension was found to be 2 and 1 in the case of the SERRS bands and the plasma lines, respectively. This result is coherent with our assumption of two SERRS signal components, the ®rst from porphyrins adsorbed on single colloidal particles and the second from the porphyrins in Ag aggregates. The number of aggregates inside the excited volume is of several orders lower in comparison with the single particles, and due to the ¯uctuation of this number an increased noise of the later component is expected. This noise should correlate with the noise of the plasma lines intensities, originated in the elastic scattering and therefore being dependent in the same way on the presence of the aggregates inside the excited volume. Besides the short time ¯uctuations, the time evolution of the SERRS spectra exhibited also slow alterations related to the development of the aggregate formation. These long time variations were equalised by means of a cubic ®t of the Vij dependence on i. Correlation coef®cients between the plasma line signal and the FA spectral constituents obtained for the SERRS bands were then calculated according to
131
the formula N X
r pl; poj
i1
Vi1pl 2 V1pl C Vijpo 2 Vjpo N´s 1pl ´s jpo
2
j 1; 2 where pl is related to the plasma lines and po to the porphyrin SERRS spectrum. The obtained correlation coef®cients were r pl; po1 20:08 and r pl; po2 0:31: This manifests that the porphyrin SERRS spectra really consist of two contributions, intensity ¯uctuations of the ®rst not being correlated and of the second being correlated with the ¯uctuations of the plasma line intensities, i.e. with the Rayleigh scattering ¯uctuations. The subspectra themselves represent, however, only abstract mathematical constructs. Isolated real SERRS spectra Zn n; n 1; 2 corresponding to the particular spectral contributions can be expressed as a linear combination of relevant subspectra Zn n
M X j1
Cnj Sj n
3
The `rotational' matrix C was found by using criteria of minimal or maximal correlation of the Z1 and Z2 contents, respectively, with the ¯uctuations of the plasma lines intensity. Finally, complete spectra (not only segments) were constructed from the original set of experimental spectra employing the linear transformation which is equivalent to that implied for segments from Eqs. (1) and (3) Zn n
M N X Cnj X V Y n Wj i1 ij i j1
n 1; 2
4
The two isolated spectra are shown in Fig. 1. We interpret the ®rst (non-correlated) spectrum as SERRS of CuTMPyP adsorbed on single colloidal particles and the second (correlated) as SERRS coming from CuTMPyP in large colloidal aggregates. The positions of the CuTMPyP SERRS bands are practically identical in both spectra, but the mutual ratios of the band intensities differ remarkably (compare, e.g. the 1363 and 1638 cm 21 bands).
132
P. Praus et al. / Journal of Molecular Structure 565±566 (2001) 129±132
4. Conclusions An evaluation of the correlation between ¯uctuations of the plasma line signal and of the SERRS bands was carried out employing factor analysis. This allowed the SERRS spectra of two SERRSactive formations, i.e. from porphyrin molecules adsorbed on single colloidal particles and those adsorbed in aggregates, to be isolated. This result demonstrates principal ability of Raman correlation spectroscopy to resolve Raman spectra coming from either small or large objects levitating in liquid. Acknowledgements This work was supported by the Grant Agency
of the Czech Republic (project No. 202/00/P069) and the Czech Ministry of Education (project No. VS 97 113).
References [1] E.L. Elson, D. Magde, Biopolymers 13 (1974) 1. [2] P. Praus, J. Bok, J. SÏteÏpaÂnek, SPIE 1403 (1990) 76. [3] W. Schrof, J. Klingler, S. Rozouvan, D. Horn, in: A.N. Heyns (Ed.), Proceedings of 16th ICORS, Wiley, Cape Town, 1998, pp. 724±727. [4] M. ProchaÂzka, P. MojzesÏ, B. VlckovaÂ, P.-Y. Turpin, J. Phys. Chem. B 101 (1997) 3161. [5] E.R. Malinowski, Factor Analysis in Chemistry, Wiley, New York, 1991.
www.elsevier.nl/locate/molstruc
In¯uence of Ag colloid aggregation on porphyrin SERRS spectra monitored via Raman correlation spectroscopy P. Praus*, M. ProchaÂzka, J. SÏteÏpaÂnek, J. Bok Institute of Physics, Charles University, Ke Karlovu 5, CZ-121 16 Prague 2, Czech Republic Received 31 August 2000; accepted 29 September 2000
Abstract Correlation spectroscopy principle was applied to study Raman signal of the SERS active system consisting of Ag colloid and copper(II) 5,10,15,20-tetrakis (1-methyl-4-pyridyl) porphyrin. A multichannel Raman spectrometer was used to record sets of consecutive Raman spectra with sampling times of tenths or units of seconds. SERRS spectra of two SERS-active formations assigned to the porphyrin molecules on the surface of Ag particles and in the aggregates were isolated by means of the statistical noise analysis. It has been proved, that Raman correlation spectroscopy can be employed to resolve complete Raman spectra coming from either small or large objects levitating in liquid. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Raman correlation spectroscopy; SERRS; Factor analysis
1. Introduction The technique of correlation spectroscopy enables an optical signal from sources the number of which ¯uctuates randomly inside the inspected volume to be distinguished by means of a statistical noise analysis. This concept has been initially introduced in absorption and ¯uorescence [1] spectroscopy. The ¯uctuations of particles in the excited volume can, however, also cause a substantial increase of the Raman signal noise [2]. The ®rst application of Raman correlation spectroscopy, when colloidal suspensions of particles with carotene were studied, was presented two years ago [3]. Two optical ®bres placed in the back focal plane of the spectrometer fed Raman signal from two spectral bands to a couple of photomultipliers, and the correlation of the output signals was monitored. Due to a parallel detection, a CCD system enables * Corresponding author. E-mail address: [email protected] (P. Praus).
to record entire wide spectral regions using sampling times of tenths or units of seconds. These times correspond well to the typical correlation times of solid particles levitating in the excited volume of nonstirred aqueous samples [2]. It indicates a prospect of separating the solid-particle Raman spectra from those of the ªsolutionº. Our contribution concerns an application of this approach to resolve the SERRS spectra of porphyrin molecules adsorbed either on single Ag colloidal particles and or on Ag particles aggregates. 2. Experimental SERS-active system was prepared by addition of copper(II) 5,10,15,20-tetrakis(1-methyl-4-pyridyl)porphyrin (CuTMPyP) to borohydride-reduced Ag colloid. Final concentration of CuTMPyP in the system was ,2 £ 10 27 M, ensuring only slight colloid aggregation [4].
0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(00)00782-1
130
P. Praus et al. / Journal of Molecular Structure 565±566 (2001) 129±132
Fig. 1. (A) Single SERRS spectrum of Ag colloid/CuTMPyP system. (B) Isolated spectrum of CuTMPyP on single colloidal particles (the ¯uctuations of the SERRS signal are not correlated with those of the plasma lines). (C) Spectrum of the porphyrins in aggregates (intensity ¯uctuations correlated with those of the plasma lines). Plasma lines are marked by asterisks.
SERRS spectra were obtained by using a multichannel Raman spectrometer equipped with a Monospec-600 monochromator (60 cm focal length, 1200 grooves/mm grating) and a liquid-N2 cooled CCD detector system (Princeton Instruments) based on an EEV 1024 £ 256 detection element. Spectra were excited by the 457.9 nm Ar 1 laser line. Average laser power at the sample in 1 ml standard Raman cell was about 25 mW. The SERRS signal was collected in a right angle scattering geometry. No bandpass ®lter was used to eliminate Ar 1 plasma lines. One thousand frames of SERRS spectra were recorded
consecutively with 0.5 s exposition time. This exposition time should correspond to the ¯uctuations correlation interval of the signal from SERRS-active inhomogeneous particles. Comparable correlation interval has been observed previously on a similar inhomogeneous system [2]. 3. Statistical analysis of SERRS spectra The spectra contained both the porphyrin SERRS bands and the plasma lines (Fig. 1). Each spectrum
P. Praus et al. / Journal of Molecular Structure 565±566 (2001) 129±132
was ®rst divided into segments covering exclusively or the SERRS bands or the plasma lines (the spectral regions with overlap of both spectral features were omitted), and the two sets of spectral fragments were then separately treated by a factor analysis (FA) [5]. FA (`singular value decomposition' algorithm) provides set of orthonormal subspectra S, matrix of orthonormal coef®cients V and vector of singular numbers W. Particular experimental spectrum Yi n can then be approximated as
Yi n
M X j1
Vij Wj Sj n
1
The factor dimension M represents the number of independent relevant components resolvable in the analysed spectral set. Its value can be derived from the decrease of the signal-to-noise ratio, diminution of the singular number values, or subsidence of the residual error [5] with increasing j. The factor dimension was found to be 2 and 1 in the case of the SERRS bands and the plasma lines, respectively. This result is coherent with our assumption of two SERRS signal components, the ®rst from porphyrins adsorbed on single colloidal particles and the second from the porphyrins in Ag aggregates. The number of aggregates inside the excited volume is of several orders lower in comparison with the single particles, and due to the ¯uctuation of this number an increased noise of the later component is expected. This noise should correlate with the noise of the plasma lines intensities, originated in the elastic scattering and therefore being dependent in the same way on the presence of the aggregates inside the excited volume. Besides the short time ¯uctuations, the time evolution of the SERRS spectra exhibited also slow alterations related to the development of the aggregate formation. These long time variations were equalised by means of a cubic ®t of the Vij dependence on i. Correlation coef®cients between the plasma line signal and the FA spectral constituents obtained for the SERRS bands were then calculated according to
131
the formula N X
r pl; poj
i1
Vi1pl 2 V1pl C Vijpo 2 Vjpo N´s 1pl ´s jpo
2
j 1; 2 where pl is related to the plasma lines and po to the porphyrin SERRS spectrum. The obtained correlation coef®cients were r pl; po1 20:08 and r pl; po2 0:31: This manifests that the porphyrin SERRS spectra really consist of two contributions, intensity ¯uctuations of the ®rst not being correlated and of the second being correlated with the ¯uctuations of the plasma line intensities, i.e. with the Rayleigh scattering ¯uctuations. The subspectra themselves represent, however, only abstract mathematical constructs. Isolated real SERRS spectra Zn n; n 1; 2 corresponding to the particular spectral contributions can be expressed as a linear combination of relevant subspectra Zn n
M X j1
Cnj Sj n
3
The `rotational' matrix C was found by using criteria of minimal or maximal correlation of the Z1 and Z2 contents, respectively, with the ¯uctuations of the plasma lines intensity. Finally, complete spectra (not only segments) were constructed from the original set of experimental spectra employing the linear transformation which is equivalent to that implied for segments from Eqs. (1) and (3) Zn n
M N X Cnj X V Y n Wj i1 ij i j1
n 1; 2
4
The two isolated spectra are shown in Fig. 1. We interpret the ®rst (non-correlated) spectrum as SERRS of CuTMPyP adsorbed on single colloidal particles and the second (correlated) as SERRS coming from CuTMPyP in large colloidal aggregates. The positions of the CuTMPyP SERRS bands are practically identical in both spectra, but the mutual ratios of the band intensities differ remarkably (compare, e.g. the 1363 and 1638 cm 21 bands).
132
P. Praus et al. / Journal of Molecular Structure 565±566 (2001) 129±132
4. Conclusions An evaluation of the correlation between ¯uctuations of the plasma line signal and of the SERRS bands was carried out employing factor analysis. This allowed the SERRS spectra of two SERRSactive formations, i.e. from porphyrin molecules adsorbed on single colloidal particles and those adsorbed in aggregates, to be isolated. This result demonstrates principal ability of Raman correlation spectroscopy to resolve Raman spectra coming from either small or large objects levitating in liquid. Acknowledgements This work was supported by the Grant Agency
of the Czech Republic (project No. 202/00/P069) and the Czech Ministry of Education (project No. VS 97 113).
References [1] E.L. Elson, D. Magde, Biopolymers 13 (1974) 1. [2] P. Praus, J. Bok, J. SÏteÏpaÂnek, SPIE 1403 (1990) 76. [3] W. Schrof, J. Klingler, S. Rozouvan, D. Horn, in: A.N. Heyns (Ed.), Proceedings of 16th ICORS, Wiley, Cape Town, 1998, pp. 724±727. [4] M. ProchaÂzka, P. MojzesÏ, B. VlckovaÂ, P.-Y. Turpin, J. Phys. Chem. B 101 (1997) 3161. [5] E.R. Malinowski, Factor Analysis in Chemistry, Wiley, New York, 1991.