**4. Anodic oxides used as a SERS substrate**

Anodic oxides are the group of materials which can be described as layered material made on the top of valve metal (like Al, Ti, Zn, Sn, W, etc. [42–47]) or its alloy (like Ti6Al7Nb, FeAl, etc. [48–51]) by electrooxidation, called anodization. Nowadays it was discovered that it is also possible to anodize cupper foil which leads to production of anodic oxide layer which consists of nanobundle or nanorods [52–54]. Anodization is the only method for anodic oxide production. The main morphologies possible to obtain by anodization are:


The most popular anodic oxides are aluminum anodic oxide (AAO) and titanium anodic oxide (ATO).

AAO is a layer of nanocapillaries, perpendicular to the Al substrate, with a diameter in the range of nanometer scale (nanoporous morphology).

Two main applications of AAO are:


**67**

*An Overview of Anodic Oxides Derived Advanced Nanocomposites Substrate for Surface...*

the way of using AAO and ATO as a SERS substrate will be discussed.

one can find examples of using AAO in preparation of SERS substrate.

(AAO/Ag) directly as the SERS substrate [84–86].

to 1.79 × 107

ranging from 1 ppb to 103

distance between them.

from 8.00 × 106

77 nm [74].

The morphological features of ATO can be relatively easily controlled by parameters of titanium anodization process. ATO is mainly used as a photocatalyst (after annealing) [63], but new applications of that material are still developed. Below,

The nanofabrication of nanostructures using the anodic aluminum oxide (AAO) template is one of the methods leading to the production of uniformly distributed nanoobjects on the substrate. The AAO is system of hexagonally ordered nanocapillaries, which are vertically oriented to the substrate surface. AAO matrix is produced during an electrochemical oxidation of aluminum foil. It is possible to control geometric parameters of AAO (such as pore diameter, distance between them, matrix thickness, degree of nanopore ordering, etc.), implemented by appropriate selection of matrix production process parameters (e.g., type, concentration and temperature of electrolyte, applied voltage, anodization time) [64–71]. It provides wide possibilities of controlling the morphology and distribution of metallic nanostructures fabricated by AAO template-assisted method, which provoked the multiple use of the AAO matrix in research aimed at making SERS substrates. Below

Because of the advantages of using AAO in nanostructure fabrication (like low cost, facile, and recurrent), a lot of research groups study the possibility of utilizing these materials in SERS substrate production. An ultrathin AAO membrane (up to 100 nm thick) was used to produce nanodots, nanowires, or metallic nanowires with plasmonic properties, and samples prepared in this way were used as SERS substrates [72–79]. Successful attempts were also made to produce SERS flexible substrates using AAO matrices [20, 80, 81]. AAO was also used as a matrix for the production of composite SERS substrates [82, 83] or after coating with silver

Nguyen et al. [73] built SERS substrate based on hexagonally ordered Au nanodisks, which was deposited on quartz by ultrathin AAO used as a shadow mask. The tested molecule was methamidophos (MAP) at various concentrations

phological features of nanodisks: diameter and distance between nanostructures. The authors observed clearly the tendency of shifting the plasmon resonance to the longer wavelength with increasing the size of Au nanodisks and with decreasing the

Liu et al. also reported correlation between the size of Au nanocones and its SERS properties in rhodamine 6G (R6G) detection [74]. They prepared highly ordered Au nanocone arrays with a diameter in the range of 36–77 nm and the distance between the centers of nanocones about 100 nm. To prepare this substrate, the AAO mask was used and Au was evaporated on glass. It was experimentally proven that the spectral position of plasmon resonances was seen to slightly blueshift with increasing the nanocone size, which coincided well with the result of finite-difference time-domain (FDTD) simulation. Moreover, compared with the bulk sample, the estimated SERS enhancement factor in R6G detection was boosted

Kim et al. [76] fabricated a highly ordered array of Au nanorod arrays consisted of agglomerated nanoparticles by porous anodic aluminum oxide (AAO) templateassisted electrochemical deposition. That prepared SERS substrate was tested for 1 mM 4-methylbenzenethiol (4-MBT) detection in three systems: Au nanorod arrays with agglomerated structures after removing AAO template, smooth structures after removing AAO template, and agglomerated structures before removing AAO template. It was shown that the Au nanorod arrays with agglomerated

ppm. It was found that SERS signal depends on the mor-

as the diameter of Au nanocones increased from 36 to

*DOI: http://dx.doi.org/10.5772/intechopen.92811*

ATO can be made by anodization in two morphologies: nanoporous (similar to AAO, but in worse nanopore arrangement) or nanotubular [60–62].

#### *An Overview of Anodic Oxides Derived Advanced Nanocomposites Substrate for Surface... DOI: http://dx.doi.org/10.5772/intechopen.92811*

The morphological features of ATO can be relatively easily controlled by parameters of titanium anodization process. ATO is mainly used as a photocatalyst (after annealing) [63], but new applications of that material are still developed. Below, the way of using AAO and ATO as a SERS substrate will be discussed.

The nanofabrication of nanostructures using the anodic aluminum oxide (AAO) template is one of the methods leading to the production of uniformly distributed nanoobjects on the substrate. The AAO is system of hexagonally ordered nanocapillaries, which are vertically oriented to the substrate surface. AAO matrix is produced during an electrochemical oxidation of aluminum foil. It is possible to control geometric parameters of AAO (such as pore diameter, distance between them, matrix thickness, degree of nanopore ordering, etc.), implemented by appropriate selection of matrix production process parameters (e.g., type, concentration and temperature of electrolyte, applied voltage, anodization time) [64–71]. It provides wide possibilities of controlling the morphology and distribution of metallic nanostructures fabricated by AAO template-assisted method, which provoked the multiple use of the AAO matrix in research aimed at making SERS substrates. Below one can find examples of using AAO in preparation of SERS substrate.

Because of the advantages of using AAO in nanostructure fabrication (like low cost, facile, and recurrent), a lot of research groups study the possibility of utilizing these materials in SERS substrate production. An ultrathin AAO membrane (up to 100 nm thick) was used to produce nanodots, nanowires, or metallic nanowires with plasmonic properties, and samples prepared in this way were used as SERS substrates [72–79]. Successful attempts were also made to produce SERS flexible substrates using AAO matrices [20, 80, 81]. AAO was also used as a matrix for the production of composite SERS substrates [82, 83] or after coating with silver (AAO/Ag) directly as the SERS substrate [84–86].

Nguyen et al. [73] built SERS substrate based on hexagonally ordered Au nanodisks, which was deposited on quartz by ultrathin AAO used as a shadow mask. The tested molecule was methamidophos (MAP) at various concentrations ranging from 1 ppb to 103 ppm. It was found that SERS signal depends on the morphological features of nanodisks: diameter and distance between nanostructures. The authors observed clearly the tendency of shifting the plasmon resonance to the longer wavelength with increasing the size of Au nanodisks and with decreasing the distance between them.

Liu et al. also reported correlation between the size of Au nanocones and its SERS properties in rhodamine 6G (R6G) detection [74]. They prepared highly ordered Au nanocone arrays with a diameter in the range of 36–77 nm and the distance between the centers of nanocones about 100 nm. To prepare this substrate, the AAO mask was used and Au was evaporated on glass. It was experimentally proven that the spectral position of plasmon resonances was seen to slightly blueshift with increasing the nanocone size, which coincided well with the result of finite-difference time-domain (FDTD) simulation. Moreover, compared with the bulk sample, the estimated SERS enhancement factor in R6G detection was boosted from 8.00 × 106 to 1.79 × 107 as the diameter of Au nanocones increased from 36 to 77 nm [74].

Kim et al. [76] fabricated a highly ordered array of Au nanorod arrays consisted of agglomerated nanoparticles by porous anodic aluminum oxide (AAO) templateassisted electrochemical deposition. That prepared SERS substrate was tested for 1 mM 4-methylbenzenethiol (4-MBT) detection in three systems: Au nanorod arrays with agglomerated structures after removing AAO template, smooth structures after removing AAO template, and agglomerated structures before removing AAO template. It was shown that the Au nanorod arrays with agglomerated

*Assorted Dimensional Reconfigurable Materials*

**Fabrication technology**

angle

Nanobludgeons, vapor deposition, dynamic evaporation at an

Ag layers, physical vapor deposition

Au nanoparticles with bimodal diameter distribution (60 nm and 15 nm)—larger particles are probably colloids and smaller ones are probably vapor deposited

**Company name/SERS substrate name**

Horiba Ltd., Japan

Enhanced Spectroscopy, Inc., USA

Nanova Inc., USA

**Table 2.**

**4. Anodic oxides used as a SERS substrate**

morphologies possible to obtain by anodization are:

• Nanopores

• Nanotubes

anodic oxide (ATO).

• Nanowires/nanofibers

Two main applications of AAO are:

Anodic oxides are the group of materials which can be described as layered material made on the top of valve metal (like Al, Ti, Zn, Sn, W, etc. [42–47]) or its alloy (like Ti6Al7Nb, FeAl, etc. [48–51]) by electrooxidation, called anodization. Nowadays it was discovered that it is also possible to anodize cupper foil which leads to production of anodic oxide layer which consists of nanobundle or nanorods [52–54]. Anodization is the only method for anodic oxide production. The main

*Commercially available SERS substrates with company name, main features, and oriented price.*

**Features Price/active area**

No information about the price (4 × 3 mm or 5 × 7 mm)

No information about the price

25\$/piece

• Measurement recommendation on a spherical area 4 mm in diameter at the center of the

enhancement, depend-

• Very low durability, up to 70 hours after opening the

ing on the analyte type

• Only a laser with a wavelength

• The surface is not homogeneous, which results in lower repeat-

substrate

container • 105 –107

of 785 nm • Long shelf life

ability of analyses

The most popular anodic oxides are aluminum anodic oxide (AAO) and titanium

AAO is a layer of nanocapillaries, perpendicular to the Al substrate, with a

1.Used as a template for nanofabrication of AAO made in two-step anodization

2.Used as a protective, anticorrosion layer on the top of aluminum alloys [58, 59]

ATO can be made by anodization in two morphologies: nanoporous (similar

process, when the nanocapillaries are hexagonally arranged [55–57]

to AAO, but in worse nanopore arrangement) or nanotubular [60–62].

diameter in the range of nanometer scale (nanoporous morphology).

**66**

structures demonstrated a higher activity in SERS effect as abundant nanogaps are created uniformly by combination of hot spots caused by both agglomerated porous structures on each nanorod and inter-rod gaps. Interestingly, that SERS substrate tested before removing AAO template exhibits the lowest signal intensity.

Liu et al. described a universal method to fabricate large-area regularly ordered gold nanodots using an AAO mask [79]. The Au nanodots were patterned on different substrates such as silicon wafer, gold, and graphene substrates, and then the films were used as SERS-active cathodes in Li-O2 batteries. The discharge products on the different electrodes (graphene and gold) were analyzed, and the results indicated that the cathode reactions on these two electrodes are distinct from each other. The SERS electrodes are so sensitive that even a small amount of product formed on the nonporous, planar electrode can be detected clearly. The authors ensured that SERS-active electrode provided a uniform enhancement of the Raman spectra over the entire substrate, which is more suitable for detecting the intermediates and by-products formed on the electrode.

Several research groups work on developing flexible SERS substrates based on polymer and metallic layers, which are fabricated with the use of AAO membranes as a template for giving the shape to polymer. Zhang et al. [20] tested flexible SERS substrate made by Au sputtering on the surface of the polymer nanostructure arrays, which was made by R2R imprinting UV-NIL process where AAO was used as the mold. The thickness of sputtering Au layer was optimized, and finally the substrate with a 30-nm Au coating was chosen as it showed the highest SERS enhancement for R6G detection. The largest enhancement factor for R6G at 1366 cm<sup>−</sup><sup>1</sup> was calculated at 1.21 × 107 , and the SERS performance showed no obvious differences under different bending angles and different bending cycles. The tested substrate demonstrated excellent SERS performance and flexibility.

Zhao et al. [80] have transferred three-dimensional nanoparticle array to polymer film with the use of AAO mask. As a result, polymer-nanocone-based 3D Au nanoparticle array SERS substrate was obtained. The new class of SERS substrate presented displays a high SERS sensitivity to R6G in a concentration up to 10<sup>−</sup>12 M. The enhancement factor was calculated at 1.30 × 108 . In the other paper, Zhao et al. have reported fabrication of polymer nanopillar array with Au nanoparticle inlays used as a flexible and efficient SERS substrate [81]. The substrate was prepared by nanoimprint lithography (NIL) method using an anodic aluminum oxide (AAO) template. It was found that substrate shows high SERS sensitivity to R6G identification with concentration up to10<sup>−</sup>12 M and the enhancement factor was calculated at 8.20 × 107 . Moreover, both tested SERS substrates [80, 81] exhibit remarkable reproducibility, excellent flexibility (the SERS intensity acquired from the substrate almost remains constant after 200 bending times), and transparency in visual and infrared range, are lightweight and can be easily handled.

Chen et al. have used AAO with silver nanoparticles entrapped in the nanopores as a SERS substrate [86]. The Ag nanoparticles were seeded growth from aqueous AgNO3 solution inside the AAO pore channels and treated at elevated temperature. It was shown that Ag nanoparticles entrapped in the pore channels exhibit significant structural stability and are less susceptible to oxidation than Ag NPs on the surface of a planar substrate at high temperatures. It was revealed that Ag NPs in AAO retain substantial SERS activity after heat treatment at 500°C for 5 days and even 600°C for 3 hours. The authors claim that geometric confinement of Ag NPs in the pore channel structure of AAO has clearly played a significant role in endowing them with the enhanced stability. It seems to be likely that the stability region may well be expanded to higher temperature and longer durations with reduced dimensions of the pore channels and decreased porosity

**69**

and tenability [84].

*An Overview of Anodic Oxides Derived Advanced Nanocomposites Substrate for Surface...*

shown that SERS spectra made on as-prepared and annealed samples are basically the same, which suggests that the coalescence of adjacent Ag NPs occurring at this stage has insignificant impact on the overall hot spots available for SERS

Lim and Ng [87] compared the SERS enhancement level for 2-naphthalenothiol obtained on AAO nanofibers coated with a thin Ag layer and on a metallic thin Ag layer deposited on nanoparticles and showed that the SERS signal enhancement on a properly selected length of AAO nanofibers is almost three times higher than on a

Dan et al. used AAO as a template for Ag nanorod array fabrication and then removed the oxide and decorated nanorods by Au nanoparticles [88]. As-prepared substrate was used as a SERS substrate for R6G. The detection limit is as low as 10<sup>−</sup>16 M with excellent recyclability. According to the authors, the Raman enhancement occurs in the gap between the nanorods and the top of the nanorod bundle. The authors believe that this synthesis method can be used to prepare other metallic

Zhong et al. [89] reported fabrication of Fe nanorod arrays by AAO templateassisted method and covered them by Au nanoparticles by sputtering approach to produced large-area urchin-like Au nanospheres for testing as SERS substrate for rhodamine 6G detection. The lower concentration of detected R6G with the use of as-prepared SERS substrate was 10<sup>−</sup>12 M. Moreover, this substrate exhibits excellent reusability after being cleaned. Based on the analysis of three-dimensional finite-difference time-domain (FDTD) simulations, the strong SERS activity is mainly due to the high density of hot spots induced by the gaps between the neighboring Au-NSs. According to the authors, the urchin-like arrays exhibit promising potential as new platforms to realize novel optoelectronic devices and have a great

Despite the AAO popularity, scientists also found interest within the ATO matrix as a production part of composite SERS substrate. Arrangement of ATO nanopores or nanotubes is not so regular like in AAO, but crystal form of TiO2 shows photocatalytic properties, which can be of advantage in construction of reusable SERS substrates. Recently, there have been reports of nanocomposites composed of semiconductor and noble metal, e.g., ZnO/Ag [90] and TiO2/Ag [91–94] or protected by the patent of Polish inventors GaN/Ag [95, 96], used as SERS substrate. Below possibility of composite SERS substrate build of anodic titanium oxide and metallic

Jakubowicz et al. [93] prepared ATO membranes with Ag nano-trees embedded in the pores. They found that the Ag nano-trees have a uniform branched symmetry independent of the applied substrate. In the cited paper, the SERS activity has not been studied; however other groups reported production of similar Ag nano-trees in AAO membrane and used this as a SERS substrate for R6G identification [84]. The SERS detection limit for R6G was estimated to 1.00 × 10<sup>−</sup>11 M. Moreover, unlike the case of conventional modification of hydrophobic functional groups, the Ag dendrite-integrated AAO membrane substrates have potentials in simple, rapid, direct, and sensitive detection of pollutants with low affinity for noble metal surface, such as fluoranthene. The authors ensure that Ag-dendrites/AAO membrane exhibited excellent SERS performance with high sensitivity, good reproducibility,

Sun et al. [97] recognized Ag-coated ATO as a low-cost, uniform, and recyclable SERS-active substrate for 2-mercaptobenzoxazole (MBO) detection. The tested composite consisted of ATO made by two-step anodization of Ti foil with deposited Ag nanoparticles on the top of ATO by e-beam evaporation. The clear SERS signal

materials with outstanding structure, which can be used in many fields.

M R6G was studied. It was

*DOI: http://dx.doi.org/10.5772/intechopen.92811*

potential for more application in other fields.

nanoparticles will be discussed.

enhancement.

thin Ag layer.

of AAO. The SERS activity of tested samples with 10<sup>−</sup><sup>6</sup>

of AAO. The SERS activity of tested samples with 10<sup>−</sup><sup>6</sup> M R6G was studied. It was shown that SERS spectra made on as-prepared and annealed samples are basically the same, which suggests that the coalescence of adjacent Ag NPs occurring at this stage has insignificant impact on the overall hot spots available for SERS enhancement.

Lim and Ng [87] compared the SERS enhancement level for 2-naphthalenothiol obtained on AAO nanofibers coated with a thin Ag layer and on a metallic thin Ag layer deposited on nanoparticles and showed that the SERS signal enhancement on a properly selected length of AAO nanofibers is almost three times higher than on a thin Ag layer.

Dan et al. used AAO as a template for Ag nanorod array fabrication and then removed the oxide and decorated nanorods by Au nanoparticles [88]. As-prepared substrate was used as a SERS substrate for R6G. The detection limit is as low as 10<sup>−</sup>16 M with excellent recyclability. According to the authors, the Raman enhancement occurs in the gap between the nanorods and the top of the nanorod bundle. The authors believe that this synthesis method can be used to prepare other metallic materials with outstanding structure, which can be used in many fields.

Zhong et al. [89] reported fabrication of Fe nanorod arrays by AAO templateassisted method and covered them by Au nanoparticles by sputtering approach to produced large-area urchin-like Au nanospheres for testing as SERS substrate for rhodamine 6G detection. The lower concentration of detected R6G with the use of as-prepared SERS substrate was 10<sup>−</sup>12 M. Moreover, this substrate exhibits excellent reusability after being cleaned. Based on the analysis of three-dimensional finite-difference time-domain (FDTD) simulations, the strong SERS activity is mainly due to the high density of hot spots induced by the gaps between the neighboring Au-NSs. According to the authors, the urchin-like arrays exhibit promising potential as new platforms to realize novel optoelectronic devices and have a great potential for more application in other fields.

Despite the AAO popularity, scientists also found interest within the ATO matrix as a production part of composite SERS substrate. Arrangement of ATO nanopores or nanotubes is not so regular like in AAO, but crystal form of TiO2 shows photocatalytic properties, which can be of advantage in construction of reusable SERS substrates. Recently, there have been reports of nanocomposites composed of semiconductor and noble metal, e.g., ZnO/Ag [90] and TiO2/Ag [91–94] or protected by the patent of Polish inventors GaN/Ag [95, 96], used as SERS substrate. Below possibility of composite SERS substrate build of anodic titanium oxide and metallic nanoparticles will be discussed.

Jakubowicz et al. [93] prepared ATO membranes with Ag nano-trees embedded in the pores. They found that the Ag nano-trees have a uniform branched symmetry independent of the applied substrate. In the cited paper, the SERS activity has not been studied; however other groups reported production of similar Ag nano-trees in AAO membrane and used this as a SERS substrate for R6G identification [84]. The SERS detection limit for R6G was estimated to 1.00 × 10<sup>−</sup>11 M. Moreover, unlike the case of conventional modification of hydrophobic functional groups, the Ag dendrite-integrated AAO membrane substrates have potentials in simple, rapid, direct, and sensitive detection of pollutants with low affinity for noble metal surface, such as fluoranthene. The authors ensure that Ag-dendrites/AAO membrane exhibited excellent SERS performance with high sensitivity, good reproducibility, and tenability [84].

Sun et al. [97] recognized Ag-coated ATO as a low-cost, uniform, and recyclable SERS-active substrate for 2-mercaptobenzoxazole (MBO) detection. The tested composite consisted of ATO made by two-step anodization of Ti foil with deposited Ag nanoparticles on the top of ATO by e-beam evaporation. The clear SERS signal

*Assorted Dimensional Reconfigurable Materials*

ates and by-products formed on the electrode.

demonstrated excellent SERS performance and flexibility.

enhancement factor was calculated at 8.20 × 107

up to 10<sup>−</sup>12 M. The enhancement factor was calculated at 1.30 × 108

calculated at 1.21 × 107

easily handled.

structures demonstrated a higher activity in SERS effect as abundant nanogaps are created uniformly by combination of hot spots caused by both agglomerated porous structures on each nanorod and inter-rod gaps. Interestingly, that SERS substrate

Liu et al. described a universal method to fabricate large-area regularly ordered gold nanodots using an AAO mask [79]. The Au nanodots were patterned on different substrates such as silicon wafer, gold, and graphene substrates, and then the films were used as SERS-active cathodes in Li-O2 batteries. The discharge products on the different electrodes (graphene and gold) were analyzed, and the results indicated that the cathode reactions on these two electrodes are distinct from each other. The SERS electrodes are so sensitive that even a small amount of product formed on the nonporous, planar electrode can be detected clearly. The authors ensured that SERS-active electrode provided a uniform enhancement of the Raman spectra over the entire substrate, which is more suitable for detecting the intermedi-

Several research groups work on developing flexible SERS substrates based on polymer and metallic layers, which are fabricated with the use of AAO membranes as a template for giving the shape to polymer. Zhang et al. [20] tested flexible SERS substrate made by Au sputtering on the surface of the polymer nanostructure arrays, which was made by R2R imprinting UV-NIL process where AAO was used as the mold. The thickness of sputtering Au layer was optimized, and finally the substrate with a 30-nm Au coating was chosen as it showed the highest SERS enhancement for R6G detection. The largest enhancement factor for R6G at 1366 cm<sup>−</sup><sup>1</sup>

under different bending angles and different bending cycles. The tested substrate

Zhao et al. [80] have transferred three-dimensional nanoparticle array to polymer film with the use of AAO mask. As a result, polymer-nanocone-based 3D Au nanoparticle array SERS substrate was obtained. The new class of SERS substrate presented displays a high SERS sensitivity to R6G in a concentration

paper, Zhao et al. have reported fabrication of polymer nanopillar array with Au nanoparticle inlays used as a flexible and efficient SERS substrate [81]. The substrate was prepared by nanoimprint lithography (NIL) method using an anodic aluminum oxide (AAO) template. It was found that substrate shows high SERS sensitivity to R6G identification with concentration up to10<sup>−</sup>12 M and the

strates [80, 81] exhibit remarkable reproducibility, excellent flexibility (the SERS intensity acquired from the substrate almost remains constant after 200 bending times), and transparency in visual and infrared range, are lightweight and can be

Chen et al. have used AAO with silver nanoparticles entrapped in the nanopores as a SERS substrate [86]. The Ag nanoparticles were seeded growth from aqueous AgNO3 solution inside the AAO pore channels and treated at elevated temperature. It was shown that Ag nanoparticles entrapped in the pore channels exhibit significant structural stability and are less susceptible to oxidation than Ag NPs on the surface of a planar substrate at high temperatures. It was revealed that Ag NPs in AAO retain substantial SERS activity after heat treatment at 500°C for 5 days and even 600°C for 3 hours. The authors claim that geometric confinement of Ag NPs in the pore channel structure of AAO has clearly played a significant role in endowing them with the enhanced stability. It seems to be likely that the stability region may well be expanded to higher temperature and longer durations with reduced dimensions of the pore channels and decreased porosity

, and the SERS performance showed no obvious differences

was

. In the other

. Moreover, both tested SERS sub-

tested before removing AAO template exhibits the lowest signal intensity.

**68**

was observed for very low concentration of MBO like 10<sup>−</sup><sup>8</sup> M. Utilizing photocatalytic properties of titanium dioxide, the SERS substrate after using was illuminated by UV light by 20 minutes and rinsed in distilled water to remove residual ions and molecules and dried at room temperature. SERS substrate cleaned in that way was reused for MBO detection with success.

Wen et al. described ATO coupling with Au nanostructures as a reliable SERS substrate with application in medicine, for chronic myeloid leukemia drug evaluation [98]. A critical biomarker of apoptosis, caspase-3, was detected by SERS analysis in real samples with reasonable recoveries. The authors assumed that SERS substrate developed by them has a lot of advantages (like cost-effectiveness, excellent reproducibility, and high sensitivity), which endows it with promising potential in apoptosis monitoring and anticancer drug development.

Ling et al. reported the use of Ag embedded in TiO2 nanotube array as a recyclable SERS substrate for R6G molecule detection [99]. Concentration of R6G in SERS measurements was 10<sup>−</sup><sup>6</sup> M. It was found that the morphology of deposited Ag is strongly affected by the diameters of ATO nanotubes and the UV irradiation induced Ag aging process, especially the self-migration of silver along the tubular wall. The SERS activity is not proportional to the ATO diameters, and the enhancement factors on various roughened metallic surfaces strongly depend on the size, shape, distribution, and interaction of Ag nanoparticles with ATO.

In the other research, Roguska et al. [100] investigated Ag, Au, and Cu nanoclusters deposited on TiO2 nanotubes/Ti as SERS substrate for pyridine detection. The influence of ATO nanotube diameters as well as the amount of deposited metal on SERS performance was studied. It was found that SERS activity of composite substrate was strongly dependent on the amount of deposited metal on ATO; for example, above 0.06 mg Ag/cm<sup>2</sup> , the SERS intensity signal was higher than for referential substrate consisting of bulk activated Ag. Moreover, this high SERS activity of tested substrate, according to the authors, is mainly the merit of their specific morphology. The SERS activity depends also on the type of metal used and the increase in the following order Cu < <Au < Ag, wherein composite made of Cu/ TiO2-nanotube/Ti was less active than referential samples made from activated Cu bulk substrate. This observation could be attributed to an instantaneous oxidation of Cu clusters and particles on ATO surface, which result in quenching of SERS signals from the adsorbed molecules. Additionally, the highest substrate stability was observed for Au/TiO2-nanotube/Ti composite.

Huang et al. [101] prepared SERS substrate consisting of Ag nanoparticles (NPs) deposited on patterned TiO2 nanotube films through pulse-current (PC) electrodeposition. The resultant substrate exhibited particle-size-dependent as well as density-correlated UV–vis absorbance and SERS enhancement effect. It was shown that SERS signal of R6G molecules obtained on the tested sample was highly enhanced.

Differences in ATO and AAO matrices are not only in their morphology. Kudelski et al. compared SERS substrate behavior consisting of Ag nanoparticles deposited on AAO and ATO nanotubes [102] for pyridine and two various selected dyes. The recorded average SERS enhancement factors on Ag/TiO2-n/Ti and Ag/ Al2O3-n/Al composites was at least equal to the SERS enhancement factor on standard electrochemically roughened Ag substrates (i.e., about 106 –107 ) or higher by a factor of 2. The authors proven that SERS spectra for examined molecules are distinctly different for those two composite substrates. They proposed that observed differences may be caused by specific interaction between the Ag nanoparticles and the oxides, especially different location of Fermi level in the Ag nanoparticles deposited on two tested nanostructured oxides, AAO and ATO. Moreover, the authors suggested that the critical influence on the shape of the spectrum (the

**71**

*An Overview of Anodic Oxides Derived Advanced Nanocomposites Substrate for Surface...*

position of individual SERS signals) and most probably on the "chemical" part of SERS enhancement is the electronic properties of the composites, not the specific

In other paper from the same group, Pisarek et al. reported [85] the influence of the size of the nanotubes/nanopores of AAO and ATO decorated by Ag nanoparticle as well as the structure of the porous oxide layers on the SERS enhancement factor (EF) for pyridine and mercaptobenzoic acid detection. It was shown that for

cantly affects the EF, which reaches distinctly higher values at an optimal nanopore size (wherein the higher the EF, the smallest the nanotube diameter) than on a stan-

was shown that for Ag/Al2O3 NT/Al material the EF is significantly higher than that for Ti-based composite and an order of magnitude higher than that for commonly used in SERS measurements roughened silver. It was proven that porous alumina interact with Ag nano-deposits, hence affecting the electronic structure of the Ag-nanoparticles, which may be most useful in detecting small amounts of certain

Other researchers, Bao et al. [82] and Zhou et al. [83], combined advantages of using AAO matrix to fabricate ordered nanostructures with catalytic activity of TiO2. Based on the photocatalytic properties of titanium dioxide, the authors have obtained promising self-cleaning effect of adsorbed molecules from composite SERS substrates made of Ag nanowires (made by AAO template-assisted fabrication method) coated with a TiO2 layer. That composite was used as a SERS substrate as prepared as well as after cleaning realized by UV light irradiation. SERS effect was tested successfully for R6G (10<sup>−</sup>14 M) and methyl parathion (MP) (up to 10−<sup>6</sup>

both, fresh and recycled samples [83]. The authors gave reasons why tested compos-

2.Between collapsing nanowire tips, the gaps are created and they act as many

3.High aspect ratio (length/width) of Ag nanowires is a major factor to achiev-

The challenge that needs to be solved in order to use the SERS spectroscopy method on a wider scale than now is to develop a cheaper and repeatable method for the synthesis of uniformly distributed homogeneous nanostructures enabling the production of SERS substrates significantly enhancing the signal (>106

The ideal SERS substrate should be characterized by enhanced high electromagnetic field, not have a fluorescent background, and operate in a wide wavelength range and with unlimited laser power. In addition, it should have a long shelf life, be repeatable and uniform throughout its entire surface, be able to operate in different environments, and be available in different sizes. The future SERS substrates will be a commercially produced, cheap, and repeatable substrate, using new plasmonic materials (e.g., semiconductors, graphene), possible to use in portable Raman spectrometers. It is very likely that databases for the interpretation of standardized

1.Ag provided much higher SERS enhancement than Au and other metals.

"hot spots," where analyte molecules are positioned.

dard silver surface roughened by electrochemical cycling, i.e., EF > 106

), the size of the nanopores signifi-

. Moreover it

M) on

) in a

*DOI: http://dx.doi.org/10.5772/intechopen.92811*

the same amount of deposited Ag (0.02 mg/cm2

ite showed high SERS enhancement [83]:

ing high SERS enhancement.

SERS spectra will be created in the future.

**5. Future of SERS substrate**

repeatable manner.

surface area of substrate.

organic compounds.

*An Overview of Anodic Oxides Derived Advanced Nanocomposites Substrate for Surface... DOI: http://dx.doi.org/10.5772/intechopen.92811*

position of individual SERS signals) and most probably on the "chemical" part of SERS enhancement is the electronic properties of the composites, not the specific surface area of substrate.

In other paper from the same group, Pisarek et al. reported [85] the influence of the size of the nanotubes/nanopores of AAO and ATO decorated by Ag nanoparticle as well as the structure of the porous oxide layers on the SERS enhancement factor (EF) for pyridine and mercaptobenzoic acid detection. It was shown that for the same amount of deposited Ag (0.02 mg/cm2 ), the size of the nanopores significantly affects the EF, which reaches distinctly higher values at an optimal nanopore size (wherein the higher the EF, the smallest the nanotube diameter) than on a standard silver surface roughened by electrochemical cycling, i.e., EF > 106 . Moreover it was shown that for Ag/Al2O3 NT/Al material the EF is significantly higher than that for Ti-based composite and an order of magnitude higher than that for commonly used in SERS measurements roughened silver. It was proven that porous alumina interact with Ag nano-deposits, hence affecting the electronic structure of the Ag-nanoparticles, which may be most useful in detecting small amounts of certain organic compounds.

Other researchers, Bao et al. [82] and Zhou et al. [83], combined advantages of using AAO matrix to fabricate ordered nanostructures with catalytic activity of TiO2. Based on the photocatalytic properties of titanium dioxide, the authors have obtained promising self-cleaning effect of adsorbed molecules from composite SERS substrates made of Ag nanowires (made by AAO template-assisted fabrication method) coated with a TiO2 layer. That composite was used as a SERS substrate as prepared as well as after cleaning realized by UV light irradiation. SERS effect was tested successfully for R6G (10<sup>−</sup>14 M) and methyl parathion (MP) (up to 10−<sup>6</sup> M) on both, fresh and recycled samples [83]. The authors gave reasons why tested composite showed high SERS enhancement [83]:


### **5. Future of SERS substrate**

The challenge that needs to be solved in order to use the SERS spectroscopy method on a wider scale than now is to develop a cheaper and repeatable method for the synthesis of uniformly distributed homogeneous nanostructures enabling the production of SERS substrates significantly enhancing the signal (>106 ) in a repeatable manner.

The ideal SERS substrate should be characterized by enhanced high electromagnetic field, not have a fluorescent background, and operate in a wide wavelength range and with unlimited laser power. In addition, it should have a long shelf life, be repeatable and uniform throughout its entire surface, be able to operate in different environments, and be available in different sizes. The future SERS substrates will be a commercially produced, cheap, and repeatable substrate, using new plasmonic materials (e.g., semiconductors, graphene), possible to use in portable Raman spectrometers. It is very likely that databases for the interpretation of standardized SERS spectra will be created in the future.

*Assorted Dimensional Reconfigurable Materials*

reused for MBO detection with success.

SERS measurements was 10<sup>−</sup><sup>6</sup>

example, above 0.06 mg Ag/cm<sup>2</sup>

was observed for Au/TiO2-nanotube/Ti composite.

was observed for very low concentration of MBO like 10<sup>−</sup><sup>8</sup>

lytic properties of titanium dioxide, the SERS substrate after using was illuminated by UV light by 20 minutes and rinsed in distilled water to remove residual ions and molecules and dried at room temperature. SERS substrate cleaned in that way was

Wen et al. described ATO coupling with Au nanostructures as a reliable SERS substrate with application in medicine, for chronic myeloid leukemia drug evaluation [98]. A critical biomarker of apoptosis, caspase-3, was detected by SERS analysis in real samples with reasonable recoveries. The authors assumed that SERS substrate developed by them has a lot of advantages (like cost-effectiveness, excellent reproducibility, and high sensitivity), which endows it with promising

Ling et al. reported the use of Ag embedded in TiO2 nanotube array as a recyclable SERS substrate for R6G molecule detection [99]. Concentration of R6G in

Ag is strongly affected by the diameters of ATO nanotubes and the UV irradiation induced Ag aging process, especially the self-migration of silver along the tubular wall. The SERS activity is not proportional to the ATO diameters, and the enhancement factors on various roughened metallic surfaces strongly depend on the size,

In the other research, Roguska et al. [100] investigated Ag, Au, and Cu nanoclusters deposited on TiO2 nanotubes/Ti as SERS substrate for pyridine detection. The influence of ATO nanotube diameters as well as the amount of deposited metal on SERS performance was studied. It was found that SERS activity of composite substrate was strongly dependent on the amount of deposited metal on ATO; for

referential substrate consisting of bulk activated Ag. Moreover, this high SERS activity of tested substrate, according to the authors, is mainly the merit of their specific morphology. The SERS activity depends also on the type of metal used and the increase in the following order Cu < <Au < Ag, wherein composite made of Cu/ TiO2-nanotube/Ti was less active than referential samples made from activated Cu bulk substrate. This observation could be attributed to an instantaneous oxidation of Cu clusters and particles on ATO surface, which result in quenching of SERS signals from the adsorbed molecules. Additionally, the highest substrate stability

Huang et al. [101] prepared SERS substrate consisting of Ag nanoparticles (NPs) deposited on patterned TiO2 nanotube films through pulse-current (PC) electrodeposition. The resultant substrate exhibited particle-size-dependent as well as density-correlated UV–vis absorbance and SERS enhancement effect. It was shown that SERS signal of R6G molecules obtained on the tested sample was highly

Differences in ATO and AAO matrices are not only in their morphology. Kudelski et al. compared SERS substrate behavior consisting of Ag nanoparticles deposited on AAO and ATO nanotubes [102] for pyridine and two various selected dyes. The recorded average SERS enhancement factors on Ag/TiO2-n/Ti and Ag/ Al2O3-n/Al composites was at least equal to the SERS enhancement factor on stan-

factor of 2. The authors proven that SERS spectra for examined molecules are distinctly different for those two composite substrates. They proposed that observed differences may be caused by specific interaction between the Ag nanoparticles and the oxides, especially different location of Fermi level in the Ag nanoparticles deposited on two tested nanostructured oxides, AAO and ATO. Moreover, the authors suggested that the critical influence on the shape of the spectrum (the

dard electrochemically roughened Ag substrates (i.e., about 106

M. It was found that the morphology of deposited

, the SERS intensity signal was higher than for

–107

) or higher by a

potential in apoptosis monitoring and anticancer drug development.

shape, distribution, and interaction of Ag nanoparticles with ATO.

M. Utilizing photocata-

**70**

enhanced.

Taking into account all the requirements for an ideal SERS substrate, which are mentioned above, the anodic oxide-based nanocomposites as SERS substrate can become an efficient and popular product. Nowadays, the possibility of applying this material as a highly active SERS substrate is in the research phase, and the results are very promising. The prospective role of advanced anodic oxide based nanocomposites as SERS substrate can be attributed to the following features:


### **6. Conclusion**

Despite the undoubted advantages and potentially large possibilities of the widespread use of SERS spectroscopy in many field, its progress has been hampered by the inability of scientists and industry to produce SERS substrates with high sensitivity, stability, and repeatability. Currently commercially available SERS substrates are characterized by obtaining enhancement of 105 –107 and durability between several days and 6 months. The above limitations appear due to the relatively small control over the morphology of substrate structures at the nanoscale, which are responsible for the observed enhancement of SERS [103].

It is important to have the reproducible and homogeneous distribution of nanostructures over the entire surface of the SERS substrate, while obtaining large areas that meet these conditions. The next step is to find technology to transfer these results from the laboratory to the industrial scale providing some technological problems. The use of conventional nanostructures in fabrication methods becomes less attractive because they are relatively expensive, time-consuming, difficult, and not effective for areas larger than 1 mm<sup>2</sup> . The use of anodic aluminum oxide template for the production of nanostructures, despite promising results at the nanoscale, is fraught with many problems arising when attempting to increase the scale of production, including a complicated, multi-stage, and difficult process for producing ultrathin membranes; ordering structures only in the domain area; and difficulties in obtaining repetitive systems of nanostructure. On the other hand, the outstanding array structure made by AAO template-assisted method has a promising practical application in SERS field, so this direction will be probably developed in the future. Also using the ATO for SERS substrate production seems to be a promising solution: the possibility of combining relatively ordered nanopores/ nanotubes with its photocatalytic properties to utilize SERS substrates self-cleaning effect gives a big perspective of application on that material. In the future probably the composite SERS substrates based on AAO/ATO and Ag or Au will be commercially available.

**73**

**Author details**

Marta Michalska-Domańska

provided the original work is properly cited.

Institute of Optoelectronics, Military University of Technology, Warsaw, Poland

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: marta.michalska@wat.edu.pl

*An Overview of Anodic Oxides Derived Advanced Nanocomposites Substrate for Surface...*

This work was financed by the National Centre for Research and Development (Poland) under program LIDER IX (Nr LIDER/50/0199/L-9/17/NCBR/2018). M. Michalska-Domańska cordially acknowledges financial support from the Polish Ministry of Science and Higher Education (Scholarship for Young, Outstanding Researchers 2016–2019, agreement no. 1013/E-410/STYP/11/2016). The author thanks Dr. P. Nyga for the help in finding information about commercially available

*DOI: http://dx.doi.org/10.5772/intechopen.92811*

**Acknowledgements**

SERS substrates.

*An Overview of Anodic Oxides Derived Advanced Nanocomposites Substrate for Surface... DOI: http://dx.doi.org/10.5772/intechopen.92811*
