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Rationally designed multifunctional plasmonic nanostructures for surface-enhanced Raman spectroscopy: a review
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Rep. Prog. Phys. 77 116502 (http://iopscience.iop.org/0034-4885/77/11/116502) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 193.140.28.22 This content was downloaded on 07/11/2014 at 02:09
Please note that terms and conditions apply.
Reports on Progress in Physics Rep. Prog. Phys. 77 (2014) 116502 (22pp)
doi:10.1088/0034-4885/77/11/116502
Review Article
Rationally designed multifunctional plasmonic nanostructures for surface-enhanced Raman spectroscopy: a review Wei Xie and Sebastian Schlücker Faculty of Chemistry, University of Duisburg-Essen, D-45141 Essen, Germany E-mail: [email protected] Received 30 August 2012, revised 3 September 2014 Accepted for publication 8 September 2014 Published 6 November 2014
Invited by Masud Mansuripur Abstract
Rationally designed multifunctional plasmonic nanostructures efficiently integrate two or more functionalities into a single entity, for example, with both plasmonic and catalytic activity. This review article is focused on their synthesis and use in surface-enhanced Raman scattering (SERS) as a molecular spectroscopic technique with high sensitivity, fingerprint specificity, and surface selectivity. After a short tutorial on the fundamentals of Raman scattering and SERS in particular, applications ranging from chemistry (heterogeneous catalysis) to biology and medicine (diagnostics/imaging, therapy) are summarized. Keywords: plasmonic nanoparticles, SERS, surface enhanced Raman scattering (Some figures may appear in colour only in the online journal)
1. Introduction
[14–26]. Plasmonic NPs can also be used for high-density optical data information storage owing to their small dimensions and geometry-dependent LSPR frequencies [17–19]. In addition, the LSPR frequency is also sensitive to the surrounding medium of the NPs, which can be exploited for sensing the local environment [20, 21]. Biomedical applications of plasmonic NPs comprise their use in diagnostics and imaging as labels and contrast agents [21–24] as well as for therapy. Specifically, plasmonic NPs with LSPRs in the ‘biological window’ in the near infrared (NIR) can be used for photothermal therapy of cancers [25, 26]. Due to their optical activity by supporting LSPRs, noble metal colloids are frequently employed for surface-enhanced molecular spectroscopies such as surface-enhanced Raman spectroscopy (SERS) [27–32], for example, in order to monitor chemical processes at surfaces (see figure 1). Bi- and multifunctional plasmonic NPs integrate two or more functions into a single entity. Typical examples for monometallic bifunctional plasmonic NPs are Au nanorods (NRs) since they exhibit both plasmonic and photothermal
Due to their small size and the resulting high surface-to-volume ratio, nanoparticles (NPs) have physical and chemical properties which are distinct from their corresponding bulk materials. In particular, noble metal NPs such as gold (Au) and silver (Ag) NPs have fascinating optical properties since they support localized surface plasmon resonances (LSPRs). The plasmonic properties of metal NPs and their assemblies depend on parameters such as chemical composition, size, shape, and interparticle spacing. Plasmonic NPs in general are central building blocks in nanoscience [1–5]. In the past decade, numerous different types of plasmonic NPs have been synthesized, characterized and employed in a diverse range of applications. In the field of energy conversion, for example, plasmonically active NPs can facilitate the harvesting of light for solar cells [6–10] or contribute to the conversion of solar to chemical energy via plasmonenhanced water splitting [11–13]. Related applications such as plasmon-driven photocatalysis have recently attracted attention 0034-4885/14/116502+22$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Review Article
Rep. Prog. Phys. 77 (2014) 116502
contains the chemical information of Raman scattering. In the model of the harmonic oscillator, it solely depends on the force constant k and the reduced mass µ of the system ω R = k / µ . For small displacements around the equilibrium position, i.e. q ≈ q0, the polarizability α can be approximated by a Taylor expansion around the equilibrium position q0. Using the expansion only up to the linear term q yields:
(
)
⎛ ∂α ⎞ α = α (q ) = α0 + ⎜ ⎟ ⋅ q ⎝ ∂q ⎠0
(4)
Inserting equation (3) into equation (4) and inserting the resulting expression, together with equation (2), into equation (1) gives—by using the addition theorem for cosα · cosβ = ½{cos (α − β) + cos (α + β)}—the following expression for the induced dipole moment: Figure 1. Chemical processes on the surface of plasmonic NPs can be monitored by using surface-enhanced molecular spectroscopies.
μ = α0E0 cosω0t +
activity [33] as well as Au nanocages for drug delivery with a plasmonic carrier [34]. Hybrid multifunctional NPs for SERS applications must contain at least one plasmonically active metal, while the other component(s) add(s) novel functions to the hybrid NP. For instance, Fe2O3/Au hybrid NPs exhibit both magnetic and plasmonic activities [35]. In this review, we highlight selected applications of bi/multifunctional SERS-active NPs for chemical (sections 3.1 and 3.2) and biomedical (sections 3.3 and 3.4) applications.
t+
The Raman effect is an inelastic light scattering process by which the frequency of the incident light (ω0) is modified by the scattering system. This leads to frequency-shifted radiation with both lower (ωS ω0, anti-Stokes Raman scattering). The incident radiation induces a dipole moment in the molecule:
E ⃗ = E0⃗ cos ω0t
(1)
q = q0 cos ω Rt
(5b)
SERS combines vibrational Raman scattering from molecules with plasmonics of metal nanostructures. The excitation of a dipolar LSPR in a gold NP is shown in figure 2(a) [28]. The frequency of the surface electron oscillation against the restoring force of the lattice ions is related to material properties such as the dielectric function of the metal, the geometry/shape of the NP, and also its local dielectric environment [36–38]. The magnitude of the induced dipole moment μind is determined by the polarizability of the metal NP αmetal and the incident electric field strength E0:
(2)
The same applies to the vibrational amplitude (displacement) of the molecule:
μ = μR (ω0 ) + μS (ω0 − ω R ) + μaS (ω0 + ω R )
2.2. Surface-enhanced Raman scattering
where α is the polarizability tensor of rank 2. The latter may be represented as a polarizability ellipsoid, which reflects the ability of the molecule’s electron cloud to respond to an external electric field. For the electric component E ⃗ of the incident electromagnetic radiation, a plane wave description (harmonic Ansatz) is used:
(5a)
where the first term is Rayleigh scattering, the second one is Stokes–Raman scattering, and the third one is anti-Stokes Raman scattering. Within this classical picture, the different intensities of Stokes and anti-Stokes contributions cannot be explained. This requires quantum chemical considerations (discrete vibrational levels with vibrational quantum numbers). Since the majority of the molecules is in the vibrational ground state at room temperature, the intensity of Stokes–Raman scattering—the initial state is the vibrational ground state and the final state is the first vibrationally excited state—is stronger than for anti-Stokes–Raman scattering, which starts in the first vibrationally excited state and ends in the vibrational ground state. Therefore, the majority of Raman experiments are performed on the Stokes side.
2.1. Classical description of Raman scattering
μ ⃗ = αE ⃗
1 ⎛ ∂α ⎞ ⎜ ⎟ q0E0 cos (ω0 + ω R ) t 2 ⎝ ∂q ⎠0
Equation (5a) contains three dipole contributions oscillating at different frequencies:
2. Brief tutorial on surface-enhanced Raman scattering
1 ⎛ ∂α ⎞ ⎜ ⎟ q0E0 cos (ω0 − ω R ) 2 ⎝ ∂q ⎠0
(3)
where ω R is the vibrational frequency. The latter is characteristic for functional groups within molecules and therefore
μind (ω0 ) = αmetalE0 (ω0 )
(6)
This oscillating dipole μind (ω0) emits radiation at the same frequency of the incident light (green arrows in figure 2(b)). An 2
Review Article
Rep. Prog. Phys. 77 (2014) 116502
SERS inherits all the advantages of conventional Raman spectroscopy, but overcomes the disadvantage of low scattering cross sections due to the signal increase via plasmon-assisted scattering. This makes SERS a very powerful analytical technique for a broad range of applications [29, 39–40]. SERS (equation (8)) is characterized not only by its high sensitivity, fingerprint specificity, and multiplex detection capability (narrow bands of vibrational Raman scattering), but also its surface selectivity [41–42]. The electric field strength of dipolar radiation scales with
E (r ) ∝ r − 3
(9)
where r is the distance between the scatterer and the metal surface. Using the |E|4 approximation for SERS, the following SERS distance dependence results:
increased local electric field Eloc is generated near the metal surface when the incident light frequency matches the frequency of the surface-electron oscillation (plasmon frequency). In other words: the LSPR-supporting metal nanostructure acts as a nanoantenna via elastic scattering upon resonant excitation by light. Let us now consider a molecule experiencing this increased local electric field Eloc(ω0). According to equation (1), the dipole moment induced in the molecule by Eloc in the presence of the nanoantenna is larger compared to an excitation with the smaller field amplitude E0 in the absence of the nanoantenna. The generated Stokes–Raman scattering (orange arrows in figure 1(b)) itself can now be elastically scattered off the LSPR-supporting nanostructure. In other words, both the incident radiation as well as the Stokes–Raman scattering are both enhanced via elastic scattering off the nanoantenna. The overall SERS intensity of SERS therefore depends on both the ‘incoming’ (ω0) and the ‘outgoing’ field (ω0 − ωR): ISERS ~ Iloc (ω0 ) Iloc (ω0 − ω R ) = Eloc (ω0 ) 2 Eloc (ω0 − ω R ) 2
3. Applications of multifunctional plasmonic nanostructrures 3.1. Label-free monitoring of heterogeneous catalysis
Catalysis is a central topic in chemistry since many chemical reactions would either not occur at all or would be too slow in the absence of a catalyst. The reactants and the catalyst can be present either in the same phase (homogeneous catalysis) or in different phases (heterogeneous catalysis). For the design and synthesis of improved catalysts, a fundamental mechanistic understanding of the molecular processes occurring during the catalytic transformation is required. Vibrational spectroscopy and Raman scattering in particular provide the necessary chemical specificity. SERS as a specialized Raman technique additionally provides the required sensitivity. Both SERS and heterogeneous catalysis take place on a surface, of either a plasmonic substrate or a catalyst. However, SERS monitoring of heterogeneous catalysis is challenging since SERS activity and catalytic activity are usually disjunct from each other, e.g. large Au NPs are SERS-active but catalytically inactive and Pt NPs are catalytically active but SERS-inactive. To overcome these limitations, the catalyst material and the plasmonically active metal must be co-localized. For instance, heterogeneous catalysis of molecules adsorbed on a Au nanoplate can be monitored by using a plasmonically active AFM tip [43]. The AFM tip was coated with Ag and then placed in contact with the reactant molecules to provide the required near-field
(7)
For ω0 » ωR (which is a good approximation only for the NIR), the SERS intensity can be approximated by the fourth power of the electric field amplitude of the incident radiation:
ISERS ~ Elocω0 4
(10)
Therefore, strongly enhanced Raman scattering only occurs in very close vicinity (ca.
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About
Contact us
My IOPscience
Rationally designed multifunctional plasmonic nanostructures for surface-enhanced Raman spectroscopy: a review
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Rep. Prog. Phys. 77 116502 (http://iopscience.iop.org/0034-4885/77/11/116502) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 193.140.28.22 This content was downloaded on 07/11/2014 at 02:09
Please note that terms and conditions apply.
Reports on Progress in Physics Rep. Prog. Phys. 77 (2014) 116502 (22pp)
doi:10.1088/0034-4885/77/11/116502
Review Article
Rationally designed multifunctional plasmonic nanostructures for surface-enhanced Raman spectroscopy: a review Wei Xie and Sebastian Schlücker Faculty of Chemistry, University of Duisburg-Essen, D-45141 Essen, Germany E-mail: [email protected] Received 30 August 2012, revised 3 September 2014 Accepted for publication 8 September 2014 Published 6 November 2014
Invited by Masud Mansuripur Abstract
Rationally designed multifunctional plasmonic nanostructures efficiently integrate two or more functionalities into a single entity, for example, with both plasmonic and catalytic activity. This review article is focused on their synthesis and use in surface-enhanced Raman scattering (SERS) as a molecular spectroscopic technique with high sensitivity, fingerprint specificity, and surface selectivity. After a short tutorial on the fundamentals of Raman scattering and SERS in particular, applications ranging from chemistry (heterogeneous catalysis) to biology and medicine (diagnostics/imaging, therapy) are summarized. Keywords: plasmonic nanoparticles, SERS, surface enhanced Raman scattering (Some figures may appear in colour only in the online journal)
1. Introduction
[14–26]. Plasmonic NPs can also be used for high-density optical data information storage owing to their small dimensions and geometry-dependent LSPR frequencies [17–19]. In addition, the LSPR frequency is also sensitive to the surrounding medium of the NPs, which can be exploited for sensing the local environment [20, 21]. Biomedical applications of plasmonic NPs comprise their use in diagnostics and imaging as labels and contrast agents [21–24] as well as for therapy. Specifically, plasmonic NPs with LSPRs in the ‘biological window’ in the near infrared (NIR) can be used for photothermal therapy of cancers [25, 26]. Due to their optical activity by supporting LSPRs, noble metal colloids are frequently employed for surface-enhanced molecular spectroscopies such as surface-enhanced Raman spectroscopy (SERS) [27–32], for example, in order to monitor chemical processes at surfaces (see figure 1). Bi- and multifunctional plasmonic NPs integrate two or more functions into a single entity. Typical examples for monometallic bifunctional plasmonic NPs are Au nanorods (NRs) since they exhibit both plasmonic and photothermal
Due to their small size and the resulting high surface-to-volume ratio, nanoparticles (NPs) have physical and chemical properties which are distinct from their corresponding bulk materials. In particular, noble metal NPs such as gold (Au) and silver (Ag) NPs have fascinating optical properties since they support localized surface plasmon resonances (LSPRs). The plasmonic properties of metal NPs and their assemblies depend on parameters such as chemical composition, size, shape, and interparticle spacing. Plasmonic NPs in general are central building blocks in nanoscience [1–5]. In the past decade, numerous different types of plasmonic NPs have been synthesized, characterized and employed in a diverse range of applications. In the field of energy conversion, for example, plasmonically active NPs can facilitate the harvesting of light for solar cells [6–10] or contribute to the conversion of solar to chemical energy via plasmonenhanced water splitting [11–13]. Related applications such as plasmon-driven photocatalysis have recently attracted attention 0034-4885/14/116502+22$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Review Article
Rep. Prog. Phys. 77 (2014) 116502
contains the chemical information of Raman scattering. In the model of the harmonic oscillator, it solely depends on the force constant k and the reduced mass µ of the system ω R = k / µ . For small displacements around the equilibrium position, i.e. q ≈ q0, the polarizability α can be approximated by a Taylor expansion around the equilibrium position q0. Using the expansion only up to the linear term q yields:
(
)
⎛ ∂α ⎞ α = α (q ) = α0 + ⎜ ⎟ ⋅ q ⎝ ∂q ⎠0
(4)
Inserting equation (3) into equation (4) and inserting the resulting expression, together with equation (2), into equation (1) gives—by using the addition theorem for cosα · cosβ = ½{cos (α − β) + cos (α + β)}—the following expression for the induced dipole moment: Figure 1. Chemical processes on the surface of plasmonic NPs can be monitored by using surface-enhanced molecular spectroscopies.
μ = α0E0 cosω0t +
activity [33] as well as Au nanocages for drug delivery with a plasmonic carrier [34]. Hybrid multifunctional NPs for SERS applications must contain at least one plasmonically active metal, while the other component(s) add(s) novel functions to the hybrid NP. For instance, Fe2O3/Au hybrid NPs exhibit both magnetic and plasmonic activities [35]. In this review, we highlight selected applications of bi/multifunctional SERS-active NPs for chemical (sections 3.1 and 3.2) and biomedical (sections 3.3 and 3.4) applications.
t+
The Raman effect is an inelastic light scattering process by which the frequency of the incident light (ω0) is modified by the scattering system. This leads to frequency-shifted radiation with both lower (ωS ω0, anti-Stokes Raman scattering). The incident radiation induces a dipole moment in the molecule:
E ⃗ = E0⃗ cos ω0t
(1)
q = q0 cos ω Rt
(5b)
SERS combines vibrational Raman scattering from molecules with plasmonics of metal nanostructures. The excitation of a dipolar LSPR in a gold NP is shown in figure 2(a) [28]. The frequency of the surface electron oscillation against the restoring force of the lattice ions is related to material properties such as the dielectric function of the metal, the geometry/shape of the NP, and also its local dielectric environment [36–38]. The magnitude of the induced dipole moment μind is determined by the polarizability of the metal NP αmetal and the incident electric field strength E0:
(2)
The same applies to the vibrational amplitude (displacement) of the molecule:
μ = μR (ω0 ) + μS (ω0 − ω R ) + μaS (ω0 + ω R )
2.2. Surface-enhanced Raman scattering
where α is the polarizability tensor of rank 2. The latter may be represented as a polarizability ellipsoid, which reflects the ability of the molecule’s electron cloud to respond to an external electric field. For the electric component E ⃗ of the incident electromagnetic radiation, a plane wave description (harmonic Ansatz) is used:
(5a)
where the first term is Rayleigh scattering, the second one is Stokes–Raman scattering, and the third one is anti-Stokes Raman scattering. Within this classical picture, the different intensities of Stokes and anti-Stokes contributions cannot be explained. This requires quantum chemical considerations (discrete vibrational levels with vibrational quantum numbers). Since the majority of the molecules is in the vibrational ground state at room temperature, the intensity of Stokes–Raman scattering—the initial state is the vibrational ground state and the final state is the first vibrationally excited state—is stronger than for anti-Stokes–Raman scattering, which starts in the first vibrationally excited state and ends in the vibrational ground state. Therefore, the majority of Raman experiments are performed on the Stokes side.
2.1. Classical description of Raman scattering
μ ⃗ = αE ⃗
1 ⎛ ∂α ⎞ ⎜ ⎟ q0E0 cos (ω0 + ω R ) t 2 ⎝ ∂q ⎠0
Equation (5a) contains three dipole contributions oscillating at different frequencies:
2. Brief tutorial on surface-enhanced Raman scattering
1 ⎛ ∂α ⎞ ⎜ ⎟ q0E0 cos (ω0 − ω R ) 2 ⎝ ∂q ⎠0
(3)
where ω R is the vibrational frequency. The latter is characteristic for functional groups within molecules and therefore
μind (ω0 ) = αmetalE0 (ω0 )
(6)
This oscillating dipole μind (ω0) emits radiation at the same frequency of the incident light (green arrows in figure 2(b)). An 2
Review Article
Rep. Prog. Phys. 77 (2014) 116502
SERS inherits all the advantages of conventional Raman spectroscopy, but overcomes the disadvantage of low scattering cross sections due to the signal increase via plasmon-assisted scattering. This makes SERS a very powerful analytical technique for a broad range of applications [29, 39–40]. SERS (equation (8)) is characterized not only by its high sensitivity, fingerprint specificity, and multiplex detection capability (narrow bands of vibrational Raman scattering), but also its surface selectivity [41–42]. The electric field strength of dipolar radiation scales with
E (r ) ∝ r − 3
(9)
where r is the distance between the scatterer and the metal surface. Using the |E|4 approximation for SERS, the following SERS distance dependence results:
increased local electric field Eloc is generated near the metal surface when the incident light frequency matches the frequency of the surface-electron oscillation (plasmon frequency). In other words: the LSPR-supporting metal nanostructure acts as a nanoantenna via elastic scattering upon resonant excitation by light. Let us now consider a molecule experiencing this increased local electric field Eloc(ω0). According to equation (1), the dipole moment induced in the molecule by Eloc in the presence of the nanoantenna is larger compared to an excitation with the smaller field amplitude E0 in the absence of the nanoantenna. The generated Stokes–Raman scattering (orange arrows in figure 1(b)) itself can now be elastically scattered off the LSPR-supporting nanostructure. In other words, both the incident radiation as well as the Stokes–Raman scattering are both enhanced via elastic scattering off the nanoantenna. The overall SERS intensity of SERS therefore depends on both the ‘incoming’ (ω0) and the ‘outgoing’ field (ω0 − ωR): ISERS ~ Iloc (ω0 ) Iloc (ω0 − ω R ) = Eloc (ω0 ) 2 Eloc (ω0 − ω R ) 2
3. Applications of multifunctional plasmonic nanostructrures 3.1. Label-free monitoring of heterogeneous catalysis
Catalysis is a central topic in chemistry since many chemical reactions would either not occur at all or would be too slow in the absence of a catalyst. The reactants and the catalyst can be present either in the same phase (homogeneous catalysis) or in different phases (heterogeneous catalysis). For the design and synthesis of improved catalysts, a fundamental mechanistic understanding of the molecular processes occurring during the catalytic transformation is required. Vibrational spectroscopy and Raman scattering in particular provide the necessary chemical specificity. SERS as a specialized Raman technique additionally provides the required sensitivity. Both SERS and heterogeneous catalysis take place on a surface, of either a plasmonic substrate or a catalyst. However, SERS monitoring of heterogeneous catalysis is challenging since SERS activity and catalytic activity are usually disjunct from each other, e.g. large Au NPs are SERS-active but catalytically inactive and Pt NPs are catalytically active but SERS-inactive. To overcome these limitations, the catalyst material and the plasmonically active metal must be co-localized. For instance, heterogeneous catalysis of molecules adsorbed on a Au nanoplate can be monitored by using a plasmonically active AFM tip [43]. The AFM tip was coated with Ag and then placed in contact with the reactant molecules to provide the required near-field
(7)
For ω0 » ωR (which is a good approximation only for the NIR), the SERS intensity can be approximated by the fourth power of the electric field amplitude of the incident radiation:
ISERS ~ Elocω0 4
(10)
Therefore, strongly enhanced Raman scattering only occurs in very close vicinity (ca.
