4. Composite thin film: effect of metal NPs' concentration

Composite colloidal films are an option for having a substrate with the advantages of rough, periodic and plasmonic material. Its performance depends on the concentration, shape, and size of the metal NPs; type of probe molecule; and thickness of the film. Consequently, the large amount of variables involved makes difficult to establish the trend that may follow a good SERS substrate.

Au and Ag are the most plasmonic materials used to boost the SERS signal. However, it is easier to incorporate Au on crystal colloids than Ag. From the several interesting reports about the synthesis and characterization of composite colloidal thin films with Au, we can highlight the following aspects [7, 17, 32]:


#### 4.1 Evidence of incorporation of metal NPs

Recently, we reported SERS-substrate films with the advantage of a rough surface provided by a periodic array of SiO2 spheres and Au NPs [32]. The synthesis of colloidal SiO2 spheres was performed following the methods of Stöber [33] and Razo [5], while Au NP colloid was prepared adopting the method introduced by Turkevich [34, 35]. The size of silica spheres and Au NPs is 275 and 22 nm, respectively. Besides, the surface plasmon resonance of the NPs is located at 522 nm (see Ref. [32]). The as-synthesized SiO2 spheres and Au NPs were used to obtain composite films with low, medium, and high volume concentrations of Au NPs (composite 1, 2, and 3, respectively), accordingly to the procedure reported by Cong et al. [36], and that previously has been employed by some of us to prepare the opal/ Fe3O4 colloidal crystal [37]. The films were deposited on a glass substrate, and to make evidence of their functionality as SERS substrates, methylene blue (MB) was employed as the molecular probe. An EF of the order of 10<sup>5</sup> was reached with the composite 3.

In Figure 7, UV–vis absorbance (A) spectrum of the films made of SiO2 spheres before and after infiltration (composite 1, 2, and 3) is presented. The blue non–soft lines correspond to the measured spectra. For the opal, the peak is attributed to the PB, which originates from the diffraction of the 3D ordered structure of the colloidal crystal. With the naked eye, a wider and asymmetric spectrum of the composites compared to that of the opal is appreciated. To enquire the effect of each component of the composite on the overall spectrum, a deconvolution analysis may be carried out. Considering a Gaussian curve associated to the silica opal and other

Micrographs by SEM of films made of SiO2 spheres before (a) and after infiltration of Au NPs (b). (c) and (d) show the center to center distance distribution histograms between adjacent spheres. The average distance corresponds to 278 � 20 nm (opal) and 275 � 20 nm, respectively. Image modified with permission from

=2σ<sup>2</sup>

where E is a fitting value that moves the curve along the dependent variable axis; Bopal and CAu are real values related to the highest point of each Gaussian curve;

opal <sup>þ</sup> <sup>C</sup>Au exp �ð Þ <sup>λ</sup> � <sup>λ</sup>Au

2 =2σ<sup>2</sup>

(10)

Au ,

to the Au NPs, the general expression is of the form:

<sup>A</sup>ð Þ¼ <sup>λ</sup> <sup>E</sup> <sup>þ</sup> <sup>B</sup>opal exp � <sup>λ</sup> � <sup>λ</sup>opal <sup>2</sup>

Metallo-Dielectric Colloidal Films as SERS Substrate DOI: http://dx.doi.org/10.5772/intechopen.90313

Figure 6.

Ref. [32].

101

Figure 6(a) exhibits the surface of the SiO2 opal where some internal planes are appreciated. Despite the presence of some vacancies, an FCC array is recognized. Scanning electron microscopy (SEM) image of composite 3 reveals that Au NPs are located at the interstitial sites (see Figure 6(b)). Moreover, the center-to-center distance between adjacent spheres in the composite is almost the same as that of the bare SiO2 opal (panels (c) and (d) of Figure 6). Hence, the lattice of the opal is not distorted even using a relatively high volume concentration of Au NPs. As the concentration of Au NPs increases, more NPs cover the SiO2 spheres and the consequent visual effect is SiO2 spheres of smaller diameter (as it is well noticed in Figure 6(b)). A similar tendency has been perceived with nonspherical Au NPs, and at low concentrations, Au nanorods reside in the voids, but as the concentration increases, they start to cover the surface of the spheres [17].

Metallo-Dielectric Colloidal Films as SERS Substrate DOI: http://dx.doi.org/10.5772/intechopen.90313

Figure 6.

asymmetry of the spectrum and difference in position of the gap center is presumably due to the presence of defects and polydispersity of the synthesized thin film.

Composite colloidal films are an option for having a substrate with the advantages of rough, periodic and plasmonic material. Its performance depends on the concentration, shape, and size of the metal NPs; type of probe molecule; and thickness of the film. Consequently, the large amount of variables involved makes

Au and Ag are the most plasmonic materials used to boost the SERS signal. However, it is easier to incorporate Au on crystal colloids than Ag. From the several interesting reports about the synthesis and characterization of composite colloidal

• When the size of the plasmonic NPs is smaller than the size of the dielectric

• The photonic band of the composite shows a red shift, a reduction of its width

Recently, we reported SERS-substrate films with the advantage of a rough surface provided by a periodic array of SiO2 spheres and Au NPs [32]. The synthesis of colloidal SiO2 spheres was performed following the methods of Stöber [33] and Razo [5], while Au NP colloid was prepared adopting the method introduced by Turkevich [34, 35]. The size of silica spheres and Au NPs is 275 and 22 nm, respectively. Besides, the surface plasmon resonance of the NPs is located at 522 nm (see Ref. [32]). The as-synthesized SiO2 spheres and Au NPs were used to obtain composite films with low, medium, and high volume concentrations of Au NPs (composite 1, 2, and 3, respectively), accordingly to the procedure reported by Cong et al. [36], and that previously has been employed by some of us to prepare the opal/ Fe3O4 colloidal crystal [37]. The films were deposited on a glass substrate, and to make evidence of their functionality as SERS substrates, methylene blue (MB) was employed as the molecular probe. An EF of the order of 10<sup>5</sup> was reached with the

Figure 6(a) exhibits the surface of the SiO2 opal where some internal planes are appreciated. Despite the presence of some vacancies, an FCC array is recognized. Scanning electron microscopy (SEM) image of composite 3 reveals that Au NPs are located at the interstitial sites (see Figure 6(b)). Moreover, the center-to-center distance between adjacent spheres in the composite is almost the same as that of the bare SiO2 opal (panels (c) and (d) of Figure 6). Hence, the lattice of the opal is not distorted even using a relatively high volume concentration of Au NPs. As the concentration of Au NPs increases, more NPs cover the SiO2 spheres and the consequent visual effect is SiO2 spheres of smaller diameter (as it is well noticed in Figure 6(b)). A similar tendency has been perceived with nonspherical Au NPs, and at low concentrations, Au nanorods reside in the voids, but as the concentration

• Even for large loads of Au NPs, evidence of the plasmon resonance is not

4. Composite thin film: effect of metal NPs' concentration

Nanorods and Nanocomposites

difficult to establish the trend that may follow a good SERS substrate.

thin films with Au, we can highlight the following aspects [7, 17, 32]:

spheres, the lattice of the opal is not distorted.

and intensity upon increasing NP doping level.

4.1 Evidence of incorporation of metal NPs

composite 3.

100

noticeable in the reflectance spectrum of the composite.

increases, they start to cover the surface of the spheres [17].

Micrographs by SEM of films made of SiO2 spheres before (a) and after infiltration of Au NPs (b). (c) and (d) show the center to center distance distribution histograms between adjacent spheres. The average distance corresponds to 278 � 20 nm (opal) and 275 � 20 nm, respectively. Image modified with permission from Ref. [32].

In Figure 7, UV–vis absorbance (A) spectrum of the films made of SiO2 spheres before and after infiltration (composite 1, 2, and 3) is presented. The blue non–soft lines correspond to the measured spectra. For the opal, the peak is attributed to the PB, which originates from the diffraction of the 3D ordered structure of the colloidal crystal. With the naked eye, a wider and asymmetric spectrum of the composites compared to that of the opal is appreciated. To enquire the effect of each component of the composite on the overall spectrum, a deconvolution analysis may be carried out. Considering a Gaussian curve associated to the silica opal and other to the Au NPs, the general expression is of the form:

$$A(\lambda) = E + B\_{\text{opal}} \exp\left(-\left(\lambda - \lambda\_{\text{opal}}\right)^2 / 2\sigma\_{\text{opal}}^2\right) + C\_{\text{Au}} \exp\left(-\left(\lambda - \lambda\_{\text{Au}}\right)^2 / 2\sigma\_{\text{Au}}^2\right),\tag{10}$$

where E is a fitting value that moves the curve along the dependent variable axis; Bopal and CAu are real values related to the highest point of each Gaussian curve;

As the concentration of Au NPs in the opal film increases, the center of the respective Gaussian curve (red line) is red shifted, compared to the surface plasmon resonance of the colloidal solution of Au NPs (located at 522 nm). Meanwhile, the center of the respective Gaussian curve assigned to the silica signal (green line) is slightly red shifted. The last statement agrees with the apparent reduction of the size of the silica spheres when their surface becomes to be covered by Au NPs. These indicatives confirm the increasing Au NPs' content in the composites.

Opals with inclusions of Au NPs offer several of the following features:

• The SERS signal is more intense compared to a disordered template.

• The uniformity/periodicity of the Au NPs'spatial distribution may be

• The roughness increases the effective surface of the SERS substrate.

• Less than a milligram of NPs is required to coat a surface of tens of square

Electric field intensity of (a) 6, (b) 9, and (c) 17 Au NPs at the intersite of three SiO2 spheres. The lower panels

• A thin layer of Au NPs is enough to increase the SERS signal.

4.2 Effect of metal NPs' load on the SERS EF

Metallo-Dielectric Colloidal Films as SERS Substrate DOI: http://dx.doi.org/10.5772/intechopen.90313

• They are simple and inexpensive to prepare.

controlled by the template.

millimeters.

Figure 8.

103

(d), (e), and (f) are the same systems but with Ag NPs.

[16, 17, 32]:

Figure 7.

UV–Vis absorbance spectrum of the films made of SiO2 spheres before and after infiltration with low (composite 1), medium (composite 2), and high (composite 3) degree of Au NP loading. Blue non–soft line is the experimental measurement; red and blue lines are the Gaussian curves associated to the Au and silica arrays, respectively; blue soft line is the sum of blue and green lines.


#### Table 2.

Gaussian fitting parameters for absorbance bands.

λopal and λAu are the central wavelengths; and σopal and σAu are the respective FWHM. The corresponding fitting values are presented in Table 2. The addition of the two Gaussians give rise to a distribution (blue soft line) that fits the experimental distribution, see panels of Figure 7. The red and green lines are the Gaussian curves attributed to the contribution of Au and silica arrays in the composite, respectively.

SiO2 film spectrum is very well adjusted to a Gaussian with a peak around 568 nm and a FWHM of 26 nm (see Figure 7). Meanwhile, when relatively low, medium, or high Au NPs concentration is incorporated in the opal, an asymmetrical and wide absorbance spectrum is noticed. At low concentration (composite 1), apparently still dominates the band assigned to the SiO2 array and the outcome is a distribution with a slightly negative asymmetry. At medium (composite 2) and high (composite 3) concentration, it seems that the Au NP signal starts to predominate and the result is a distribution with a positive asymmetry.

Metallo-Dielectric Colloidal Films as SERS Substrate DOI: http://dx.doi.org/10.5772/intechopen.90313

As the concentration of Au NPs in the opal film increases, the center of the respective Gaussian curve (red line) is red shifted, compared to the surface plasmon resonance of the colloidal solution of Au NPs (located at 522 nm). Meanwhile, the center of the respective Gaussian curve assigned to the silica signal (green line) is slightly red shifted. The last statement agrees with the apparent reduction of the size of the silica spheres when their surface becomes to be covered by Au NPs. These indicatives confirm the increasing Au NPs' content in the composites.
