**2.2.1 Photopolymerized patterns of PMMA/Fe2O3**

Solutions of methyl methacrylate (MMA) monomer mixed with colloidal γ-Fe2O3 NPs in the presence of a photoinitiator IRGACURE®1700 were prepared in chloroform at concentrations of 89.5 wt.%, 10 wt.% and 0.5 wt.%, respectively. The Fe2O3 NPs were previously prepared in chloroform, by modifying a wet-chemical synthetic approach previously reported (Sun et al 2004), in order to obtain hydrophobic-capped Fe2O3 spherical particles with a mean diameter of 10 nm. Oleic acid, oleylamine, and hexadecane-1,2-diol were used as capping molecules of the produced NPs. All solutions were stirred and left in the dark for few minutes to equilibrate. 200 μL of each solution were spin-coated at 1000 rpm for 20 s, and subsequently ~40 μL of the same solution were drop-casted onto glass substrates. For the alignment of the nanoparticles, the system was subjected to a homogeneous magnetic field (~160 mT), produced by two permanent magnets, applied parallel to the substrate during the deposition, evaporation and photopolymerization processes.

The patterned PMMA/Fe2O3 nanocomposites were obtained by irradiating all samples with the third harmonic of a pulsed Nd:YAG laser (Quanta-Ray GCR 190, Spectra Physics) with an energy density of 10.5 mJ·cm-2 (λ=355 nm, pulse duration =4-6 ns, repetition rate=10 Hz) for 90 minutes, using aluminum photomasks with different geometries. After the photopolymerization, each sample was washed three times with methanol to remove unreacted monomer and photoinitiator and then dried under ambient dark conditions for 2 days to achieve complete solvent evaporation. In Figure 4 are demonstrated characteristic patterns created upon irradiation of the spin coated and subsequently drop casted MMA/Fe2O3 NPs solution through a photomask of two parallel lines in the mm range.

During the photopolymerization, a homogeneous magnetic field produced by two permanent magnets was applied to the samples, at saturated chloroform atmosphere as shown in Figure 5. All samples prepared with or without the application of the magnetic field were studied under an optical microscope. Figure 6 presents samples with 10 wt. % concentration of NPs. The analysis reveals that in the samples created without the application of the magnetic field (Figures 6a and 6b), the NPs form aggregates randomly distributed in the film. The application of a magnetic field produced by two magnets leads

Laser-Based Lithography for Polymeric Nanocomposite Structures 297

a b

c d

Fig. 6. Optical microscopy images of PMMA/Fe2O3 films formed without the application of a magnetic field (a) before and (b) after the photopolymerization, and with the application of a magnetic field (c) before and (d) after the photopolymerization and washing. In the panel (d) is demonstrated the edge of the photopolymerized film after the washing.

A detailed topographic study of the photolithographically produced patterns with or without the application of the magnetic field was conducted using AFM (Figures 7a-7d). Figures 7a and 7b show the topography of samples produced in the absence of a magnetic field. The nanofillers of these samples do not show any particular alignment, as expected, but rather organize themselves in order to form aggregates of different sizes distributed all over the surface, in agreement with the results of the optical microscopy examination. On the contrary, the AFM images of the samples realized under the magnetic field produced by two magnets show the presence of parallel structures aligned along the direction of the external magnetic field (Figures 7c and 7d). In particular, parallel lines with thickness of ~1 µm and length of tens of microns clearly appear both in the 2D and in the 3D topography images. The images also demonstrate the presence of bigger aggregated structures with width around 5 µm, which can be attributed to aligned NWs that in some areas of the sample get very closely packed, due to the high NPs concentration. It is very likely that the NWs formation inside the photopatterned polymer are facilitated by the initial organization of the NPs into bigger clusters, as demonstrated already in Figures 7a and 7b. These clusters exhibit higher magnetic moments than the single Fe2O3 NPs, resulting in an increased response to the external magnetic field that leads eventually to chain formation (Lalatonne

The formation of magnetic NWs inside and on the surface of polymeric patterned structures opens up the possibility of various applications of these systems, related to the oriented growth and patterning of molecules bound on the NWs, and to the effect of this binding on the magnetic properties of the NWs. In particular, specially designed structures can be created where various biological molecules such as DNA, proteins, or cells can be bound on

et al 2004).

to NPs aligned along the magnetic field lines, forming parallel NWs (Figure 6c and 6d). By these two figures is demonstrated that the formation of magnetic NWs when the samples are placed under the magnetic field occurs both before and after the photopolymerization procedure, with the latter leaving unaffected the geometry of the wires. In particular, Figure 6d shows that the area of the sample that was covered by the mask (bottom part of the picture) is not polymerized and thus, after washing with methanol, the composite is removed and no aligned structures are left onto the substrate.

Fig. 4. Photolithographically created lines of mm-dimensions of PMMA/γ-Fe2O3 NPs nanocomposites.

Fig. 5. Schematization of the experimental set-up where a homogeneous magnetic field is applied parallel to the samples.

to NPs aligned along the magnetic field lines, forming parallel NWs (Figure 6c and 6d). By these two figures is demonstrated that the formation of magnetic NWs when the samples are placed under the magnetic field occurs both before and after the photopolymerization procedure, with the latter leaving unaffected the geometry of the wires. In particular, Figure 6d shows that the area of the sample that was covered by the mask (bottom part of the picture) is not polymerized and thus, after washing with methanol, the composite is

Fig. 4. Photolithographically created lines of mm-dimensions of PMMA/γ-Fe2O3 NPs

Fig. 5. Schematization of the experimental set-up where a homogeneous magnetic field is

removed and no aligned structures are left onto the substrate.

nanocomposites.

applied parallel to the samples.

Fig. 6. Optical microscopy images of PMMA/Fe2O3 films formed without the application of a magnetic field (a) before and (b) after the photopolymerization, and with the application of a magnetic field (c) before and (d) after the photopolymerization and washing. In the panel (d) is demonstrated the edge of the photopolymerized film after the washing.

A detailed topographic study of the photolithographically produced patterns with or without the application of the magnetic field was conducted using AFM (Figures 7a-7d). Figures 7a and 7b show the topography of samples produced in the absence of a magnetic field. The nanofillers of these samples do not show any particular alignment, as expected, but rather organize themselves in order to form aggregates of different sizes distributed all over the surface, in agreement with the results of the optical microscopy examination. On the contrary, the AFM images of the samples realized under the magnetic field produced by two magnets show the presence of parallel structures aligned along the direction of the external magnetic field (Figures 7c and 7d). In particular, parallel lines with thickness of ~1 µm and length of tens of microns clearly appear both in the 2D and in the 3D topography images. The images also demonstrate the presence of bigger aggregated structures with width around 5 µm, which can be attributed to aligned NWs that in some areas of the sample get very closely packed, due to the high NPs concentration. It is very likely that the NWs formation inside the photopatterned polymer are facilitated by the initial organization of the NPs into bigger clusters, as demonstrated already in Figures 7a and 7b. These clusters exhibit higher magnetic moments than the single Fe2O3 NPs, resulting in an increased response to the external magnetic field that leads eventually to chain formation (Lalatonne et al 2004).

The formation of magnetic NWs inside and on the surface of polymeric patterned structures opens up the possibility of various applications of these systems, related to the oriented growth and patterning of molecules bound on the NWs, and to the effect of this binding on the magnetic properties of the NWs. In particular, specially designed structures can be created where various biological molecules such as DNA, proteins, or cells can be bound on

Laser-Based Lithography for Polymeric Nanocomposite Structures 299

a thick coating. The solution was then spin-coated on a silicon wafer and subsequently left under a magnetic field (400 mT) with a vertical direction with respect to the substrate, resulting in the formation of wire like magnetic structures. These magnetic wires induce magnetic anisotropy so for specific magnetic field orientations the magnetic response is higher (Fragouli & Buonsanti et al 2010). After, the sample was irradiated with a UV lamp mask aligner for 70 sec with energy of 25 mW. Masks of square shaped pillar patterns were used, with 42 µm side and different interpillar distances varying from 14 to 77 µm. The postbake was for 1 min at 65° C and for 4 min at 95° C. Finally, the samples were developed with SU-8 developer (15min) and rinsed with isopropanol with subsequent drying under nitrogen flow. The fabricated nanocomposite pillars formed in this way, had a square side of 42 µm and 40 µm high as shown in the image recorded with Scanning Electron Microscopy

Fig. 8. SEM image of the SU-8/iron oxide nanocomposite pillar structures.

The AFM image of figure 9 demonstrates a part of the surface of a nanocomposite pillar in which the NPs are aligned vertically to the substrate. The nanoroughness shown is due to the protruding edges of the formed NWs, demonstrating that the diameters of the NWs are

The magnetic alignment of the iron oxide NPs inside the SU-8 matrix creates a magnetic anisotropy that enhances the possibility for the pillar structures to respond to an external magnetic field. The responsivity of the magnetic pillars to an external magnetic field was checked using water drops placed onto the pillars. In particular, the pillars were inclined in order to observe a difference between the right and the left part of the drops and a static magnet of maximum field strength 500 mT, was moved towards the sample with a direction parallel to it. The test was performed onto magnetic pillars with interpillar distance 14 μm. The water contact angle on these pillars is about 130°, while the values of the right and the left contact angle after the tilting of the sample are shown in table 1. After few minutes it was observed a change in the shape of the drop when the magnet was moving towards the drop, while it was recovering when the magnet was far from the substrate. This reversible difference in the shape of the drop indicates the tendency of the drop to move, which is connected with the reversible movement of the magnetic pillars responding to the external

(SEM) (Figure 8).

around 100 nm.

magnetic field.

the NWs. Such structures can be extremely useful in biological applications for the formation of biological sensors, and can be also incorporated in molecular recognition devices

Fig. 7. AFM images of PMMA/Fe2O3 films realized with or without a magnetic field. The images show the topography (a) and the phase (b) of a sample realized in the absence of a magnetic field, and the 2D (c) and 3D (d) topography images of a sample realized under the magnetic field of two magnets. In particular, the 2D and 3D topography images of sample areas with parallel lines show the existence of aligned structures on the surface of the sample.

### **2.2.2 Photopolymerized patterns of SU-8/Fe2O3**

SU-8 is a commercial biocompatible epoxy-based negative photoresist that is suitable for the microfabrication of high aspect-ratio (>20) structures. When exposed to UV light, its molecular chains crosslink causing the polymerization of the material. Here the SU-8 3050 (Microchem) was used for the fabrication of the SU-8 pillars. Fe2O3 NPs of diameter 20 nm in choloroform solution were prepared, by modifying the wet-chemical synthetic approach previously reported by Sun et al 2004. Size control was achieved by varying the concentration of the precursor and the surfactant-to-precursor molar ratio.

For the fabrication of SU-8 magnetic pillars the following steps were performed: The existing cyclopentanone solvent was evaporated from the SU-8 3050 resin. Subsequently, SU-8 was dissolved in chloroform (1:5 wt.) by sonication for 10 min and then stirring. Then, the Fe2O3 NPs (2 wt. %) were slowly added and mixed with the SU-8 solution under sonication. After obtaining a homogeneous solution, the solvent was slowly evaporated under nitrogen flow, until it was obtained a nanocomposite solution viscous enough to form

the NWs. Such structures can be extremely useful in biological applications for the formation

of biological sensors, and can be also incorporated in molecular recognition devices

a b

c d

Fig. 7. AFM images of PMMA/Fe2O3 films realized with or without a magnetic field. The images show the topography (a) and the phase (b) of a sample realized in the absence of a magnetic field, and the 2D (c) and 3D (d) topography images of a sample realized under the magnetic field of two magnets. In particular, the 2D and 3D topography images of sample areas with parallel lines show the existence of aligned structures on the surface of the sample.

SU-8 is a commercial biocompatible epoxy-based negative photoresist that is suitable for the microfabrication of high aspect-ratio (>20) structures. When exposed to UV light, its molecular chains crosslink causing the polymerization of the material. Here the SU-8 3050 (Microchem) was used for the fabrication of the SU-8 pillars. Fe2O3 NPs of diameter 20 nm in choloroform solution were prepared, by modifying the wet-chemical synthetic approach previously reported by Sun et al 2004. Size control was achieved by varying the

For the fabrication of SU-8 magnetic pillars the following steps were performed: The existing cyclopentanone solvent was evaporated from the SU-8 3050 resin. Subsequently, SU-8 was dissolved in chloroform (1:5 wt.) by sonication for 10 min and then stirring. Then, the Fe2O3 NPs (2 wt. %) were slowly added and mixed with the SU-8 solution under sonication. After obtaining a homogeneous solution, the solvent was slowly evaporated under nitrogen flow, until it was obtained a nanocomposite solution viscous enough to form

concentration of the precursor and the surfactant-to-precursor molar ratio.

**2.2.2 Photopolymerized patterns of SU-8/Fe2O3**

a thick coating. The solution was then spin-coated on a silicon wafer and subsequently left under a magnetic field (400 mT) with a vertical direction with respect to the substrate, resulting in the formation of wire like magnetic structures. These magnetic wires induce magnetic anisotropy so for specific magnetic field orientations the magnetic response is higher (Fragouli & Buonsanti et al 2010). After, the sample was irradiated with a UV lamp mask aligner for 70 sec with energy of 25 mW. Masks of square shaped pillar patterns were used, with 42 µm side and different interpillar distances varying from 14 to 77 µm. The postbake was for 1 min at 65° C and for 4 min at 95° C. Finally, the samples were developed with SU-8 developer (15min) and rinsed with isopropanol with subsequent drying under nitrogen flow. The fabricated nanocomposite pillars formed in this way, had a square side of 42 µm and 40 µm high as shown in the image recorded with Scanning Electron Microscopy (SEM) (Figure 8).

Fig. 8. SEM image of the SU-8/iron oxide nanocomposite pillar structures.

The AFM image of figure 9 demonstrates a part of the surface of a nanocomposite pillar in which the NPs are aligned vertically to the substrate. The nanoroughness shown is due to the protruding edges of the formed NWs, demonstrating that the diameters of the NWs are around 100 nm.

The magnetic alignment of the iron oxide NPs inside the SU-8 matrix creates a magnetic anisotropy that enhances the possibility for the pillar structures to respond to an external magnetic field. The responsivity of the magnetic pillars to an external magnetic field was checked using water drops placed onto the pillars. In particular, the pillars were inclined in order to observe a difference between the right and the left part of the drops and a static magnet of maximum field strength 500 mT, was moved towards the sample with a direction parallel to it. The test was performed onto magnetic pillars with interpillar distance 14 μm. The water contact angle on these pillars is about 130°, while the values of the right and the left contact angle after the tilting of the sample are shown in table 1. After few minutes it was observed a change in the shape of the drop when the magnet was moving towards the drop, while it was recovering when the magnet was far from the substrate. This reversible difference in the shape of the drop indicates the tendency of the drop to move, which is connected with the reversible movement of the magnetic pillars responding to the external magnetic field.

Laser-Based Lithography for Polymeric Nanocomposite Structures 301

Another strategy that we follow for the in situ creation of nanofillers into the polymer matrix is the UV laser irradiation of polymer films containing cadmium thiolate precursors. This method results in the spatially selective formation of cadmium sulphide (CdS) crystalline NPs in the host matrix, through a macroscopically non-destructive procedure for the matrix. Using a pulse by pulse approach, we accomplished the formation of NPs with gradually increasing dimensions, and consequently the progressive change of the emission characteristics of the formed nanocomposites. The optimized combination of irradiation wavelength with polymer matrix gives patterned nanocomposite materials incorporating

The use of polymeric nanocomposite materials is expanding to a huge range of applications since they combine the flexibility, easy processability and low cost of the polymers with the unique properties of the nanofillers. On the other hand the intrinsic insulating characteristic of the polymers limit the possibilities of using polymeric-based systems in devices where electronic conductivity is desired, like sensors, miniaturized electronic chips, etc. In order to overcome this limitation, the use of metallic NPs that exhibit very high electronic conductivity as nanofillers is proved to be a successive strategy (Gelves 2006, 2011, Huang et

Our strategy is based on the use of the Au precursor, chloroauric acid salt (HAuCl4), introduced in a transparent polymer film by immersion, and the generation of Au particles in specific areas by means of laser irradiation. The lithographically produced nanocomposite areas have tailored properties, dependent on the density and size of the produced Au NPs. The possibility to produce Au-polymeric nanocomposite materials with enhanced electrical properties, in combination with the spatial control of the specific property by introducing in situ the nanofillers in the desired areas increases enormously the potentiality of such

Chitosan (CTO) is a natural biodegradable and biocompatible polysaccharide polymer derived from chitin, a linear chain of acetylglucosamine groups, extracted from crustaceans shells and the cell walls of many fungi. CTO is fiberlike and is obtained by the deacetylation process of the natural chitin, a process that gives rise to amine groups which can be used for further functionalization (Yi et al 2005, Luther et al 2005, Su et al 2005, Zhou et al 2006, Zangmeister et al 2006). CTO is becoming widely used due to its potential polysaccharide resource and properties as non-toxicity, excellent processability, adsorption properties, hydrogel behavior, electrospinning, etc (Guibal 2005, Nirmala et al 2011). Its chemical

CTO has a hydrogel nature resulting in the tendency to absorb ambient moisture or liquids. In this work, the process was optimized and used to introduce gold precursor in CTO polymer thin films. By controlling the immersion time of the polymer film in gold precursor solution, the absorption of gold precursors is highly controlled. The CTO used in this work was purchased from Sigma Aldrich with a degree of deacetylation about 80%. Various concentrations of CTO polymer solutions (0.5%, 1% and 2% wt.) are prepared in acetic acid. CTO polymeric films on glass substrates are obtained by drop-casting or spin-coating. The use of high CTO polymer concentration allows the formation of CTO hydrogel films able to absorb gold precursor crystals behaving as a "gold precursor reservoir". The gold precursor

nanocrystals of high quality, ready to be used in various optical applications.

**3.1 Au precursor-based nanocomposite patterns** 

systems in a wide variety of applications.

structure is illustrated in figure 10.

**3.1.1 Spatially controlled in situ formation of Au NPs in chitosan** 

al 2009).

Fig. 9. AFM topography of the nanocomposite SU-8 mixed with iron oxide NPs, vertically aligned under the application of an external magnetic field.


Table 1. Contact angle values of the left and right part of the water drop when the magnet is away from the substrate (top raw) and when the magnet in coming closer (down raw). At the right column are demonstrated the two examined frames.
