Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 398–402
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Hyper-Rayleigh scattering from gold nanoparticles: Effect of size and shape K. Das a,⇑, A. Uppal a, R.K. Saini a, G.K. Varshney a, P. Mondal b, P.K. Gupta a a b
Laser Bio-Medical Applications & Instrumentation Division, Raja Ramanna Center for Advanced Technology, Indore 452013, MP, India Indus Synchrotrons Utilization Division, Raja Ramanna Center for Advanced Technology, Indore 452013, MP, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Hyper-Rayleigh scattering from five
First hyperpolarizability (b) values of gold nanoparticles: Effect of size and shape
different gold nanoparticles are compared. Star and flower shaped particles were observed to have highest hyperpolarizability. The hyper-Rayleigh scattering from the nanoparticles showed a dipolar response.
Star (core dia ~ 90 nm) Flower (core dia ~ 57 nm) Tetrapod (tip length ~ 26 nm) Rod (Aspect ratio ~2.2) Sphere ~ 20nm Sphere ~ 10nm 0
20
40
60 -25
β per particle (x 10
a r t i c l e
i n f o
Article history: Received 16 November 2013 Received in revised form 20 February 2014 Accepted 23 February 2014 Available online 12 March 2014 Keywords: Gold nanoparticles Hyper-Rayleigh scattering
100
a b s t r a c t We report hyper-Rayleigh scattering (HRS) properties of gold nanoparticles (GNPs) of five different shapes, quasi-spherical (10 and 20 nm diameter), rod (aspect ratio 2), and branched shapes, tetrapod, flower and star with 800 nm, 150 fs laser excitation. Using 10 nm spherical GNPs as reference, the first hyperpolarizability (b) values were calculated for all other shapes. Star and flower shaped GNPs have the highest hyperpolarizability (130 and 52 times higher, respectively), while rod and tetrapod shaped GNPs only have modest enhancement (7 times), which is similar to 20 nm size quasi-spherical particles. These enhancements are attributed to reduced symmetry as well as the presence of sharp tips on GNP surface. When the b values are normalized with respect to the number of atoms per particle, the flower and star shaped GNPs still have the highest hyperpolarizability values. The polar plots of vertically polarized HRS signal as a function of the angle of polarization of the incoming incident light shows two lobes, indicating that excitation is predominantly dipolar in nature although the size of some GNPs are big enough to show a quadrupolar response. It is believed that the presence of sharp tips at the surface of these large sized GNPs is responsible for the observed dipolar response. This study shows that GNPs having sharp tips might be a better candidate when their nonlinear properties are used for sensing applications. Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel./fax: +91 0731 2488436. E-mail addresses: [email protected], [email protected] (K. Das). http://dx.doi.org/10.1016/j.saa.2014.02.152 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
80
esu)
K. Das et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 398–402
399
Introduction
Results
Nanomaterials made from gold and silver generally absorb in the visible region of the spectrum due to the collective excitation of the conduction band electrons. This phenomenon is known as the surface plasmon resonance (SPR). Their non linear optical (NLO) properties are superior to organic chromophores due to their strong extinction at SPR wavelength [1–16]. The NLO properties of metallic nanoparticless are often judged by their hyperpolarizability (b) values. Recent reports have shown that NLO properties of these nanomaterials can be used for biological and chemical sensing [1,13,17–21]. For sensing applications it is desirable to have nanomaterials with high hyperpolarizability (b) values. Because gold is chemically inert, non-toxic nanomaterials made from gold are preferred. Due to the lack of a good dipole moment, hyperRayleigh scattering (HRS) is the method of choice for the measurement of the first hyperpolarizabilities of nanoparticles dispersed in a liquid solution [2–16]. HRS from a NP solution originates from the lack of centrosymmetry of the particle as demonstrated earlier for nearly spherical gold and silver nanoparticles [2–9]. In this paper we compare the suitability of five different shaped gold nanoparticles (GNPs) for sensing applications utilizing their NLO properties. The suitability was judged by comparing their first hyperpolarizability values determined by the HRS method using 800 nm, 150 fs laser excitation. It may be noted that the HRS signal is also accompanied by a strong multi-photon luminescence background. However, the narrow spectral bandwidth of the HRS signal makes it more convenient for user selected detection range and quantification which makes it a desirable property to be utilized for sensing applications.
Fig. 1 shows the TEM images of the six GNP’s used in this study. The two sets of quasi-spherical GNP’s have an average size of 10 ± 2 and 20 ± 3 nm. While the smaller particles are more spherical in shape the bigger particles are significantly irregular in shape. The star and flower shaped GNPs were observed to have pointed tips. While the flower shaped particles have average core diameter of 60 ± 15 nm, the star shaped particles have an average core diameter of 90 ± 20 nm. Tetrapods show four tips (average tip length of 26 ± 5 nm) and depending upon their relative orientation on the carbon coated copper grid, their TEM images shows two distinct shapes. Nanorods have an average aspect ratio of 2 (length: 38 ± 4 nm; breadth: 17 ± 3 nm). Table 1 summarizes the average sizes of these GNPs. The extinction spectra of these GNPs are presented in Fig. 2. The two sets of quasi-spherical GNP’s have nearly identical spectra. However for all the other shapes, significant red shifts in the spectra were observed. In addition, the spectral shape becomes asymmetric and broad, particularly for the flower shaped NPs. The spectral response of these NPs against 800 nm femtosecond laser excitation is shown in Fig. 3. The sharp peak at 400 nm corresponds to HRS which is associated with a broad background. This broad background is attributed to luminescence due to multiphoton absorption [23–33]. It is clear that HRS and luminescence intensity changes significantly with size and shape. The HRS intensity increases significantly for the tetrapod, flower and star shaped GNPs and the broad luminescence background were observed to be significant for star, tetrapod and rod shaped GNPs. The inset in Fig. 3 provides a comparison of the integrated HRS and luminescence intensities obtained from the different GNPs. For all GNPs, the luminescence background was observed to be significantly higher than the HRS signal. The HRS intensity I2x is given by:
Materials and methods The various GNPs were prepared according to methods available in literature and the preparation details are given in supporting information. The shapes and sizes of these GNPs were analyzed from their transmission electron microscope (TEM) images. The TEM was a Philips CM200 with W-filament as cathode and was used at an accelerating anode voltage of 200KV. Extinction spectra of the various GNPs were recorded using a Cintra absorption spectrophotometer having a resolution of 2 nm. HRS experiments was performed by exciting the GNP samples with 150 fs pulses centered at 800 nm obtained from a tunable Ti-Sapphire laser (Coherent Mira). The laser beam was focused by a 20 cm lens into a 1 cm path length quartz cuvette containing the nanoparticle solution. The solution was constantly stirred during the experiment to reduce any unwanted photothermal effects. The HRS from the GNPs was detected at right angle to the laser beam and a bandpass filter (transmission range 380–700 nm) was used to cut off the excitation light. The polarization state of the excitation light was selected with a rotating half-wave plate. A thin film polarizer was kept on the emission side to select the polarization state (horizontal or vertical) of the emitted HRS light. The light scattered from the nanoparticles was detected and analyzed with the LifeSpec Red TCSPC system from Edinburgh instruments. The spectral bandwidth was kept at 1 nm for all measurements. For the detection of both HRS and multi-photon induced luminescence from the GNPs, a spectral scan (380–650 nm) was taken with 1 nm step size and one second integration time per point. For b value measurements, the polarization state of the excitation light as well as HRS signal was kept horizontal [11,21,22]. For polarization resolved HRS experiments, the polarization state of the detected HRS signal was kept vertical while the polarization of the excitation light was varied from 0 to 360° where 0° corresponds to vertical polarization.
I2x ¼ GðN1 hb21 i þ N2 hb22 iÞI2x eN2 a2 l where Ix is the incident light intensity; I2x is the intensity of generated HRS signal; G is an constant which depends upon the experimental conditions, b denotes the first order hyperpolarizability, N denotes the number density and the exponential term accounts for the re-absorption of HRS signal at the second harmonic wavelength by the GNP itself. The subscripts 1 and 2 refer to solute and solvent, respectively. As noted earlier, the HRS signal rides on a background signal whose intensity increases significantly for some shapes (Fig. 3). Therefore, in order to extract the actual HRS signal, a spectral scan around 400 ± 20 nm is taken which is then fitted by a Lorentzian function. To calculate the first hyperpolarizability of the different GNPs, their number densities are required. The number densities of spheres and rods can be estimated from their sizes calculated by analysis of their TEM pictures. However the tertapod, star and flower shaped NPs are irregular in shape and therefore some approximation (regarding their shape) is necessary to get an estimate of their concentrations. For tetrapods, the tetrahedron shape was assumed and the center to tip distance was used to calculate their concentrations. For flower and star shaped particles the shape was assumed to be spherical and the core diameter is used to calculate their concentrations. It is pertinent to note that the number densities obtained in this way are not accurate, but provides a close enough estimate of their concentrations. For the first hyperpolarizability estimation, the HRS intensities of GNPs were recorded against varying incident laser power at six different GNP number densities shown in Fig. 1, supporting information. From the polynomial fits (Table 1, supporting information) it can be seen that the observed changes in HRS intensity depends quadratically on the laser power and linearly on the GNP
400
K. Das et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 398–402
Sphere (10 ± 2 nm)
Tetrapod
Sphere (20 ± 3 nm)
Star
Rod
Flower Fig. 1. Typical TEM photographs of different GNPs.
Table 1 Size analysis and first hyperpolarizability values of the GNPs used in this study. Shape
Dimension (nm)
nAu
b (1025 esu)
p b/ nAu (1028 esu)
Spherical
10 ± 2 20 ± 3 38 ± 4 (L) 17 ± 3 (B) 26 ± 5 (tip length) 57 ± 13 (core diameter) 90 ± 20 (core diameter)
139,400 470,510 741,360 201,000 9,408,800 37037000
0.6 ± 0.1 3.8 ± 0.7 4.1 ± 0.6 4.4 ± 0.8 31.0 ± 7.0 78.6 ± 15.0
1.6 ± 0.3 5.5 ± 1.0 4.8 ± 0.7 9.8 ± 1.8 10.1 ± 2.3 12.9 ± 2.5
Rod Tetrapod Flower Star
concentration. The slope obtained from a linear fit of the quadratic coefficient versus GNP number density was used to calculate the first hyperpolarizability. Fig. 4 shows the linear dependence of the quadratic coefficients with the GNP number densities. Using the equation given above, the first hyperpolarizability of 10 ± 2 nm diameter GNPs were calculated to be 0.6 1025 esu using water as an external reference (bwater = 0.56 1030 esu; taken from Ref [2]). The b values of other GNPs were calculated using the b value of 10 ± 2 nm diameter GNPs as a reference using the relation [34]:
Slope10 2 nm GNP hb2HRS i10 2 nm GNP ¼ 2 Slopeunknown GNP hbHRS iunknown GNP The calculated hyperpolarizability values of the six different GNPs are shown in Table 1. The hyperpolarizability values of 10 ± 2 and 20 ± 3 nm spherical GNPs are consistent with reported values [11], using 800 nm excitation. Star shaped GNPs have the highest hyperpolarizability followed by flower shaped GNPs (130 and 52 times higher, respectively, compared to 10 ± 2 nm spherical GNPs).
401
K. Das et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 398–402
1.2
Sphere ~20nm Flower Star Tetrapod Rod
0.8
quadratic co-efficient
Normalized extinction
Sphere ~10nm
0.4
0.0 400
500
600
700
800
900
0.01
Sphere ~20nm Flower Star Tetrapod Rod
1E-3
wavelength (nm)
1
photon counts/second
Sphere~10nm Sphere~20nm Flower Star TetrapodRod
10000
photon counts/second
Fig. 2. Normalized extinction spectra of different GNPs.
1000
5
4
10
3
10
2
10
1
10
0
Sphere Flower Tetrapod Star (~10 nm)
Rod
Sphere (~20 nm)
1000
100
450
100
Fig. 4. Variation of quadratic co-efficient (intercepts, obtained from fittings shown in Fig. 2, supporting information) versus GNP concentration and corresponding linear fits. In order to show all in the same graph, the axes are plotted in log scale.
10
10
400
10
GNP concentration (picoMolar)
500
550
600
wavelength (nm) Fig. 3. Spectral response from different GNPs used with 800 nm laser excitation. Concentrations (in terms of gold atoms) of different nanoparticles are: 0.10 mM for star, tetrapod, flower and rod; 0.5 mM for spheres (10 & 20 nm). Laser power: 500 mW. Inset: Comparison of the integrated HRS (black) and luminescence (red) intensities from different GNPs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Since an appreciable increase of GNP size might enhance the electric multipolar contribution of the observed HRS signal, we have investigated the polarization properties of the HRS signal as described in experimental section. Fig. 2 in supporting information shows polar plots of vertically polarized HRS signal as a function of the angle of polarization of excitation light for various GNPs. The polarization response of all the GNPs shows two lobes, which are similar to the one observed for pure electric dipole response from non-centrosymmetric organic molecules, as reported previously [35–38]. Discussion There is an interest on nanomaterials having high second order non-linear optical properties to be used for chemical and biological detection [1,13,17–21]. Therefore, we have prepared six different types of GNP and determined their b values with 800 nm femtosecond laser excitation. The extinction spectra of these GNPs, as expected, differ significantly because their shape as well as size varied significantly. Attempts were made to use only those GNPs which do not have significant absorption around wavelength of
excitation (800 nm) to avoid one-photon resonance enhancement of the HRS signal which will significantly affect the beta value. The beta values (Table 1) were observed to depend critically on the shape as well as size. Star and flower shaped GNPs, which are non-centrosymmetric, have the maximum b values while GNPs having centrosymmetric shapes like spherical (or nearly spherical), rod and tetrapod shape have the minimum b values. It is generally believed that the presence of sharp tips/edges in metallic nanoparticles is conducive to generate intense electric fields near their vicinity which is expected to increase their b value. It is therefore interesting to note that tetrapod shaped GNPs produce a much lower b value compared to star and flower shaped GNPs. A closer examination of the TEM pictures (Fig. 1) reveal that the tips of the tetrapods are more rounded compared to the tips of star and flower shaped GNPs and thus the electric fields generated near their tips are lower in magnitude. While the beta values reported are actually per nanoparticle, for comparison between GNPs of various shapes and sizes, a more p informative value is b/ nAu (where nAu denotes number of gold atoms per particle) [2,5,6]. When this is calculated, it is observed that flower, star and tetrapod shaped GNPs have similar beta values which is about 6–8 times higher than that of a 10 ± 2 nm spherical GNP. It must be mentioned that errors in estimating the volume of a single GNP will affect the beta values. Considering the way we have calculated the volumes, the actual beta values for star and flower shaped GNPs (excluding the tips) should be more and for tetrapod shaped GNPs it should be less. The optical responses of particles that are small compared to the wavelength of excitation (d k/10) can be described usually in the framework of the electric-dipole approximation. However, when the particle size approaches the wavelength (d k/10), the dipolar picture may no longer provide a complete description, and higher multipolar interactions should be considered. The HRS intensity therefore also consists of several contributions. The first one is the electric dipole approximation, which may arise from the NP surface. The second contribution is a multipolar contribution, such as an electric-quadrupole contribution arising from the bulk of the NP. In an earlier study [19], using 800 nm incident light, it was observed that for spherical GNP with a diameter (20 nm) much less than k, the polar plots of the vertically polarized HRS signal as a function of the angle of polarization of the incoming incident light shows two lobes, which are similar to the one for a pure electric dipole response from noncentrosymmetric organic molecules, as reported before by
402
K. Das et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 398–402
several groups [35–38]. For larger diameter GNPs (80 nm), retardation effects of the interaction of electromagnetic fields with particles cannot be neglected, and the response deviates from pure dipolar response, exhibiting a strong quadrupolar contribution. As a result the polar plots of vertically polarized HRS signal shows four lobes. In this study, the polar plots (Fig. 2, supporting information) for all GNPs show the presence of two lobes centered on 0° and 180°, indicating that the response is predominantly dipolar in origin. This is surprising given that the size of some of the GNPs studied here (especially the star and flower shaped particles) are comparable to excitation wavelength (d–k/10). This observation suggests that dipolar responses arising from the surfaces of these GNPs might be dominant over the quadrupolar responses arising from the bulk of these GNPs. It is tempting to attribute this to the presence of pointed tips at the surface, where electric field is expected to be significantly higher. For majority of the GNPs (except 10 nm spherical ones), the HRS signal was observed to ride on a background that becomes significantly higher for rod, tertapod and star shaped particles. This background, which is termed as photoluminescence, has already been reported in the past [23–33]. The observation of a photoluminescence background at energies higher than the HRS band energy (
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Hyper-Rayleigh scattering from gold nanoparticles: Effect of size and shape K. Das a,⇑, A. Uppal a, R.K. Saini a, G.K. Varshney a, P. Mondal b, P.K. Gupta a a b
Laser Bio-Medical Applications & Instrumentation Division, Raja Ramanna Center for Advanced Technology, Indore 452013, MP, India Indus Synchrotrons Utilization Division, Raja Ramanna Center for Advanced Technology, Indore 452013, MP, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Hyper-Rayleigh scattering from five
First hyperpolarizability (b) values of gold nanoparticles: Effect of size and shape
different gold nanoparticles are compared. Star and flower shaped particles were observed to have highest hyperpolarizability. The hyper-Rayleigh scattering from the nanoparticles showed a dipolar response.
Star (core dia ~ 90 nm) Flower (core dia ~ 57 nm) Tetrapod (tip length ~ 26 nm) Rod (Aspect ratio ~2.2) Sphere ~ 20nm Sphere ~ 10nm 0
20
40
60 -25
β per particle (x 10
a r t i c l e
i n f o
Article history: Received 16 November 2013 Received in revised form 20 February 2014 Accepted 23 February 2014 Available online 12 March 2014 Keywords: Gold nanoparticles Hyper-Rayleigh scattering
100
a b s t r a c t We report hyper-Rayleigh scattering (HRS) properties of gold nanoparticles (GNPs) of five different shapes, quasi-spherical (10 and 20 nm diameter), rod (aspect ratio 2), and branched shapes, tetrapod, flower and star with 800 nm, 150 fs laser excitation. Using 10 nm spherical GNPs as reference, the first hyperpolarizability (b) values were calculated for all other shapes. Star and flower shaped GNPs have the highest hyperpolarizability (130 and 52 times higher, respectively), while rod and tetrapod shaped GNPs only have modest enhancement (7 times), which is similar to 20 nm size quasi-spherical particles. These enhancements are attributed to reduced symmetry as well as the presence of sharp tips on GNP surface. When the b values are normalized with respect to the number of atoms per particle, the flower and star shaped GNPs still have the highest hyperpolarizability values. The polar plots of vertically polarized HRS signal as a function of the angle of polarization of the incoming incident light shows two lobes, indicating that excitation is predominantly dipolar in nature although the size of some GNPs are big enough to show a quadrupolar response. It is believed that the presence of sharp tips at the surface of these large sized GNPs is responsible for the observed dipolar response. This study shows that GNPs having sharp tips might be a better candidate when their nonlinear properties are used for sensing applications. Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. Tel./fax: +91 0731 2488436. E-mail addresses: [email protected], [email protected] (K. Das). http://dx.doi.org/10.1016/j.saa.2014.02.152 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
80
esu)
K. Das et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 398–402
399
Introduction
Results
Nanomaterials made from gold and silver generally absorb in the visible region of the spectrum due to the collective excitation of the conduction band electrons. This phenomenon is known as the surface plasmon resonance (SPR). Their non linear optical (NLO) properties are superior to organic chromophores due to their strong extinction at SPR wavelength [1–16]. The NLO properties of metallic nanoparticless are often judged by their hyperpolarizability (b) values. Recent reports have shown that NLO properties of these nanomaterials can be used for biological and chemical sensing [1,13,17–21]. For sensing applications it is desirable to have nanomaterials with high hyperpolarizability (b) values. Because gold is chemically inert, non-toxic nanomaterials made from gold are preferred. Due to the lack of a good dipole moment, hyperRayleigh scattering (HRS) is the method of choice for the measurement of the first hyperpolarizabilities of nanoparticles dispersed in a liquid solution [2–16]. HRS from a NP solution originates from the lack of centrosymmetry of the particle as demonstrated earlier for nearly spherical gold and silver nanoparticles [2–9]. In this paper we compare the suitability of five different shaped gold nanoparticles (GNPs) for sensing applications utilizing their NLO properties. The suitability was judged by comparing their first hyperpolarizability values determined by the HRS method using 800 nm, 150 fs laser excitation. It may be noted that the HRS signal is also accompanied by a strong multi-photon luminescence background. However, the narrow spectral bandwidth of the HRS signal makes it more convenient for user selected detection range and quantification which makes it a desirable property to be utilized for sensing applications.
Fig. 1 shows the TEM images of the six GNP’s used in this study. The two sets of quasi-spherical GNP’s have an average size of 10 ± 2 and 20 ± 3 nm. While the smaller particles are more spherical in shape the bigger particles are significantly irregular in shape. The star and flower shaped GNPs were observed to have pointed tips. While the flower shaped particles have average core diameter of 60 ± 15 nm, the star shaped particles have an average core diameter of 90 ± 20 nm. Tetrapods show four tips (average tip length of 26 ± 5 nm) and depending upon their relative orientation on the carbon coated copper grid, their TEM images shows two distinct shapes. Nanorods have an average aspect ratio of 2 (length: 38 ± 4 nm; breadth: 17 ± 3 nm). Table 1 summarizes the average sizes of these GNPs. The extinction spectra of these GNPs are presented in Fig. 2. The two sets of quasi-spherical GNP’s have nearly identical spectra. However for all the other shapes, significant red shifts in the spectra were observed. In addition, the spectral shape becomes asymmetric and broad, particularly for the flower shaped NPs. The spectral response of these NPs against 800 nm femtosecond laser excitation is shown in Fig. 3. The sharp peak at 400 nm corresponds to HRS which is associated with a broad background. This broad background is attributed to luminescence due to multiphoton absorption [23–33]. It is clear that HRS and luminescence intensity changes significantly with size and shape. The HRS intensity increases significantly for the tetrapod, flower and star shaped GNPs and the broad luminescence background were observed to be significant for star, tetrapod and rod shaped GNPs. The inset in Fig. 3 provides a comparison of the integrated HRS and luminescence intensities obtained from the different GNPs. For all GNPs, the luminescence background was observed to be significantly higher than the HRS signal. The HRS intensity I2x is given by:
Materials and methods The various GNPs were prepared according to methods available in literature and the preparation details are given in supporting information. The shapes and sizes of these GNPs were analyzed from their transmission electron microscope (TEM) images. The TEM was a Philips CM200 with W-filament as cathode and was used at an accelerating anode voltage of 200KV. Extinction spectra of the various GNPs were recorded using a Cintra absorption spectrophotometer having a resolution of 2 nm. HRS experiments was performed by exciting the GNP samples with 150 fs pulses centered at 800 nm obtained from a tunable Ti-Sapphire laser (Coherent Mira). The laser beam was focused by a 20 cm lens into a 1 cm path length quartz cuvette containing the nanoparticle solution. The solution was constantly stirred during the experiment to reduce any unwanted photothermal effects. The HRS from the GNPs was detected at right angle to the laser beam and a bandpass filter (transmission range 380–700 nm) was used to cut off the excitation light. The polarization state of the excitation light was selected with a rotating half-wave plate. A thin film polarizer was kept on the emission side to select the polarization state (horizontal or vertical) of the emitted HRS light. The light scattered from the nanoparticles was detected and analyzed with the LifeSpec Red TCSPC system from Edinburgh instruments. The spectral bandwidth was kept at 1 nm for all measurements. For the detection of both HRS and multi-photon induced luminescence from the GNPs, a spectral scan (380–650 nm) was taken with 1 nm step size and one second integration time per point. For b value measurements, the polarization state of the excitation light as well as HRS signal was kept horizontal [11,21,22]. For polarization resolved HRS experiments, the polarization state of the detected HRS signal was kept vertical while the polarization of the excitation light was varied from 0 to 360° where 0° corresponds to vertical polarization.
I2x ¼ GðN1 hb21 i þ N2 hb22 iÞI2x eN2 a2 l where Ix is the incident light intensity; I2x is the intensity of generated HRS signal; G is an constant which depends upon the experimental conditions, b denotes the first order hyperpolarizability, N denotes the number density and the exponential term accounts for the re-absorption of HRS signal at the second harmonic wavelength by the GNP itself. The subscripts 1 and 2 refer to solute and solvent, respectively. As noted earlier, the HRS signal rides on a background signal whose intensity increases significantly for some shapes (Fig. 3). Therefore, in order to extract the actual HRS signal, a spectral scan around 400 ± 20 nm is taken which is then fitted by a Lorentzian function. To calculate the first hyperpolarizability of the different GNPs, their number densities are required. The number densities of spheres and rods can be estimated from their sizes calculated by analysis of their TEM pictures. However the tertapod, star and flower shaped NPs are irregular in shape and therefore some approximation (regarding their shape) is necessary to get an estimate of their concentrations. For tetrapods, the tetrahedron shape was assumed and the center to tip distance was used to calculate their concentrations. For flower and star shaped particles the shape was assumed to be spherical and the core diameter is used to calculate their concentrations. It is pertinent to note that the number densities obtained in this way are not accurate, but provides a close enough estimate of their concentrations. For the first hyperpolarizability estimation, the HRS intensities of GNPs were recorded against varying incident laser power at six different GNP number densities shown in Fig. 1, supporting information. From the polynomial fits (Table 1, supporting information) it can be seen that the observed changes in HRS intensity depends quadratically on the laser power and linearly on the GNP
400
K. Das et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 398–402
Sphere (10 ± 2 nm)
Tetrapod
Sphere (20 ± 3 nm)
Star
Rod
Flower Fig. 1. Typical TEM photographs of different GNPs.
Table 1 Size analysis and first hyperpolarizability values of the GNPs used in this study. Shape
Dimension (nm)
nAu
b (1025 esu)
p b/ nAu (1028 esu)
Spherical
10 ± 2 20 ± 3 38 ± 4 (L) 17 ± 3 (B) 26 ± 5 (tip length) 57 ± 13 (core diameter) 90 ± 20 (core diameter)
139,400 470,510 741,360 201,000 9,408,800 37037000
0.6 ± 0.1 3.8 ± 0.7 4.1 ± 0.6 4.4 ± 0.8 31.0 ± 7.0 78.6 ± 15.0
1.6 ± 0.3 5.5 ± 1.0 4.8 ± 0.7 9.8 ± 1.8 10.1 ± 2.3 12.9 ± 2.5
Rod Tetrapod Flower Star
concentration. The slope obtained from a linear fit of the quadratic coefficient versus GNP number density was used to calculate the first hyperpolarizability. Fig. 4 shows the linear dependence of the quadratic coefficients with the GNP number densities. Using the equation given above, the first hyperpolarizability of 10 ± 2 nm diameter GNPs were calculated to be 0.6 1025 esu using water as an external reference (bwater = 0.56 1030 esu; taken from Ref [2]). The b values of other GNPs were calculated using the b value of 10 ± 2 nm diameter GNPs as a reference using the relation [34]:
Slope10 2 nm GNP hb2HRS i10 2 nm GNP ¼ 2 Slopeunknown GNP hbHRS iunknown GNP The calculated hyperpolarizability values of the six different GNPs are shown in Table 1. The hyperpolarizability values of 10 ± 2 and 20 ± 3 nm spherical GNPs are consistent with reported values [11], using 800 nm excitation. Star shaped GNPs have the highest hyperpolarizability followed by flower shaped GNPs (130 and 52 times higher, respectively, compared to 10 ± 2 nm spherical GNPs).
401
K. Das et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 398–402
1.2
Sphere ~20nm Flower Star Tetrapod Rod
0.8
quadratic co-efficient
Normalized extinction
Sphere ~10nm
0.4
0.0 400
500
600
700
800
900
0.01
Sphere ~20nm Flower Star Tetrapod Rod
1E-3
wavelength (nm)
1
photon counts/second
Sphere~10nm Sphere~20nm Flower Star TetrapodRod
10000
photon counts/second
Fig. 2. Normalized extinction spectra of different GNPs.
1000
5
4
10
3
10
2
10
1
10
0
Sphere Flower Tetrapod Star (~10 nm)
Rod
Sphere (~20 nm)
1000
100
450
100
Fig. 4. Variation of quadratic co-efficient (intercepts, obtained from fittings shown in Fig. 2, supporting information) versus GNP concentration and corresponding linear fits. In order to show all in the same graph, the axes are plotted in log scale.
10
10
400
10
GNP concentration (picoMolar)
500
550
600
wavelength (nm) Fig. 3. Spectral response from different GNPs used with 800 nm laser excitation. Concentrations (in terms of gold atoms) of different nanoparticles are: 0.10 mM for star, tetrapod, flower and rod; 0.5 mM for spheres (10 & 20 nm). Laser power: 500 mW. Inset: Comparison of the integrated HRS (black) and luminescence (red) intensities from different GNPs. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Since an appreciable increase of GNP size might enhance the electric multipolar contribution of the observed HRS signal, we have investigated the polarization properties of the HRS signal as described in experimental section. Fig. 2 in supporting information shows polar plots of vertically polarized HRS signal as a function of the angle of polarization of excitation light for various GNPs. The polarization response of all the GNPs shows two lobes, which are similar to the one observed for pure electric dipole response from non-centrosymmetric organic molecules, as reported previously [35–38]. Discussion There is an interest on nanomaterials having high second order non-linear optical properties to be used for chemical and biological detection [1,13,17–21]. Therefore, we have prepared six different types of GNP and determined their b values with 800 nm femtosecond laser excitation. The extinction spectra of these GNPs, as expected, differ significantly because their shape as well as size varied significantly. Attempts were made to use only those GNPs which do not have significant absorption around wavelength of
excitation (800 nm) to avoid one-photon resonance enhancement of the HRS signal which will significantly affect the beta value. The beta values (Table 1) were observed to depend critically on the shape as well as size. Star and flower shaped GNPs, which are non-centrosymmetric, have the maximum b values while GNPs having centrosymmetric shapes like spherical (or nearly spherical), rod and tetrapod shape have the minimum b values. It is generally believed that the presence of sharp tips/edges in metallic nanoparticles is conducive to generate intense electric fields near their vicinity which is expected to increase their b value. It is therefore interesting to note that tetrapod shaped GNPs produce a much lower b value compared to star and flower shaped GNPs. A closer examination of the TEM pictures (Fig. 1) reveal that the tips of the tetrapods are more rounded compared to the tips of star and flower shaped GNPs and thus the electric fields generated near their tips are lower in magnitude. While the beta values reported are actually per nanoparticle, for comparison between GNPs of various shapes and sizes, a more p informative value is b/ nAu (where nAu denotes number of gold atoms per particle) [2,5,6]. When this is calculated, it is observed that flower, star and tetrapod shaped GNPs have similar beta values which is about 6–8 times higher than that of a 10 ± 2 nm spherical GNP. It must be mentioned that errors in estimating the volume of a single GNP will affect the beta values. Considering the way we have calculated the volumes, the actual beta values for star and flower shaped GNPs (excluding the tips) should be more and for tetrapod shaped GNPs it should be less. The optical responses of particles that are small compared to the wavelength of excitation (d k/10) can be described usually in the framework of the electric-dipole approximation. However, when the particle size approaches the wavelength (d k/10), the dipolar picture may no longer provide a complete description, and higher multipolar interactions should be considered. The HRS intensity therefore also consists of several contributions. The first one is the electric dipole approximation, which may arise from the NP surface. The second contribution is a multipolar contribution, such as an electric-quadrupole contribution arising from the bulk of the NP. In an earlier study [19], using 800 nm incident light, it was observed that for spherical GNP with a diameter (20 nm) much less than k, the polar plots of the vertically polarized HRS signal as a function of the angle of polarization of the incoming incident light shows two lobes, which are similar to the one for a pure electric dipole response from noncentrosymmetric organic molecules, as reported before by
402
K. Das et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 398–402
several groups [35–38]. For larger diameter GNPs (80 nm), retardation effects of the interaction of electromagnetic fields with particles cannot be neglected, and the response deviates from pure dipolar response, exhibiting a strong quadrupolar contribution. As a result the polar plots of vertically polarized HRS signal shows four lobes. In this study, the polar plots (Fig. 2, supporting information) for all GNPs show the presence of two lobes centered on 0° and 180°, indicating that the response is predominantly dipolar in origin. This is surprising given that the size of some of the GNPs studied here (especially the star and flower shaped particles) are comparable to excitation wavelength (d–k/10). This observation suggests that dipolar responses arising from the surfaces of these GNPs might be dominant over the quadrupolar responses arising from the bulk of these GNPs. It is tempting to attribute this to the presence of pointed tips at the surface, where electric field is expected to be significantly higher. For majority of the GNPs (except 10 nm spherical ones), the HRS signal was observed to ride on a background that becomes significantly higher for rod, tertapod and star shaped particles. This background, which is termed as photoluminescence, has already been reported in the past [23–33]. The observation of a photoluminescence background at energies higher than the HRS band energy (
