Nanoplasmonics and metamaterials sciences are rapidly growing due to their contributions to photonic devices fabrication with applications ranging from biomedicine to photovoltaic cells. Noble metal nanoparticles incorporated into polymer matrix have great potential for such applications due to their distinctive optical properties. However, methods to indirectly incorporate metal nanoparticles into polymeric microstructures are still on demand. Here we report on the fabrication of two-photon polymerized microstructures doped with gold nanoparticles through an indirect doping process, so they do not interfere in the two-photon polymerization (2PP) process. Such microstructures present a strong emission, arising from gold nanoparticles fluorescence. The microstructures produced are potential candidates for nanoplasmonics and metamaterials devices applications and the nanoparticles production method can be applied in many samples, heated simultaneously, opening the possibility for large scale processes.
© 2012 OSA
Nanoplasmonics and metamaterials science have attracted great attention due to their potential application in photonic devices. Metamaterials, materials consisting of periodic micro- and nano-structures , provide unique optical properties allowing for the production of invisibility cloaks and negative refraction index materials  while nanoplasmonics, which regards to the manipulation of the optical properties in the nanoscale vicinity of a metal surface, has found a wide range of applications such as cancer treatments , chemical and biological sensing  and photovoltaic cells . In this context, noble metal nanoparticles have potential to tailor the optical properties of micro/nano devices, more specifically gold nanoparticles are known for their ability to enhance local electric field which might be used to enhance the material´s optical , electrical and also mechanical properties . The plasmon absorption band of metal nanoparticles is sensitive to the surrounding medium  what is desirable for applications involving micro sensors . In addition, since gold nanoparticles are biocompatible, they can be used for the fabrication of sensors for biology and medicine [8–10]. Studies reporting nanoparticles dispersed in a polymer matrix generally involve the direct insertion of nanoparticles in the resin, prior to polymerization . However, the presence of metal nanoparticles interferes with some polymerization processes, like two-photon polymerization (2PP) microfabrication. The 2PP has been used for the fabrication of tridimensional polymeric microstructures in applications ranging from biology to photonics [1, 10, 12–14]. With this technique one can achieve resolution below the diffraction limit  and increased light penetration depth, giving the possibility to produce complex tridimensional microstructures . Additionally, the 2PP can be used to fabricate doped microstructures with enhanced optical, biological and chemical properties aiming at applications in optical and photonics devices, such as fluorescent waveguides , birefringent elements , micro resonators [19, 20] and electrically conductive microstructures [21, 22]. However, only recently a few research groups begun to study the possibility of fabricating two-photon polymerized microstructures doped with metal nanoparticles [11, 23–26]. The resulting metal nanoparticle doped structures are an organic/inorganic hybrid bulk metamaterial with potential applications in nanoplasmonics.
In this paper we report on the fabrication of two-photon polymerized microstructures doped with gold nanoparticles through an indirect doping process, in which the nanoparticles are produced after polymerization, therefore not interfering with the 2PP process. Our results show that gold nanoparticles-doped microstructures display an enhancement of the polymer matrix fluorescence arising from gold nanoparticles luminescence. In addition, by exciting these nanoparticles in their plasmon absorption band, one could use the local field enhancement effect in applications such as contrast agent for the visualization of 3D microstructures  in confocal microscopy, as a tool for sensing chemical and biological analytes  in SERS (surface enhanced Raman spectroscopy) and other nanoplasmonics applications that can benefit from the local field enhancement effect.
The experimental apparatus for the fabrication of polymeric microstructures via two-photon polymerization is fully described elsewhere , but a few details will be given here. We employed a Ti: Sapphire oscillator laser operating at 82 MHz that delivers 35 fs pulses centered at 790 nm and with 40 nm bandwidth. The fabrication setup was composed by two movable mirrors, a precision motorized stage, a red LED as illumination source and a CCD camera for monitoring the fabrication. The pulsed laser was focused through a microscope objective lens into the liquid resin. In a region around the focal volume light intensity is high enough to induce two-photon absorption by the initiating species and locally polymerize the sample. A motorized stage moved the sample in the z direction (beam propagation), while a pair of galvanometric mirrors deflected the beam in the x and y directions allowing for the fabrication of tridimensional structures. After polymerization the sample was immersed in ethanol to wash away all uncured resin, leaving on the substrate only the microstructures.
The indirect doping of gold nanoparticles into the polymeric microstructures consisted of initially mixing the monomers with an aqueous solution of HAuCl4. The resin used as host for the gold nanoparticles was composed by two different triacrylate monomers, which ratio can be varied to obtain final polymerized structures with diverse mechanical properties . Tris(2-hydroxyethyl) isocyanurate triacrylate (50 wt.%) gives hardness to the microstructure while ethoxylated(6) trimethylolpropane triacrylate (50 wt.%) is responsible for reducing shrinkage upon polymerization. Both monomers were mixed in excess of ethanol. To this resin formulation we added a solution of HAuCl4 in water (2 g/l) which was mixed to the resin in a proportion of 1 ml of the solution to 2.5 g of resin mixture. The sample was left for 24 hours at 50°C for evaporation of the solvents prior to the fabrication process. To this mixture we added 3 weight % in excess of the photoinitiator ethyl-2,4,6-trimethylbenzoyl phenylphosphinate, which has been shown to be useful for two-photon absorption polymerization [28, 29].
After the two-photon polymerization fabrication and rinsing processes, the sample was submitted to thermal annealing in a specific range of temperature and time. This thermal treatment is responsible for producing gold nanoparticles in the polymeric microstructure bulk, where the last acted as a reducing agent for dispersed gold ions. Among the several thermal treatments tested, the one consisting of heating the sample up to 185°C for approximately 35 minutes yielded the best results concerning the amount of and time to nanoparticles production. Differential Scanning Calorimetry (DSC) analysis of macroscopic polymerized samples demonstrate that degradation temperature for the polymerized resins is beyond 400 °C, which is well above the thermal annealing temperature used (185 °C), preventing any thermal damage on the microstructures. The presence of the nanoparticles in the microstructures bulk was noted through a strong fluorescence not observed in the non-heated HAuCl4 doped microstructures. The optical properties of the two-photon polymerized microstructures were obtained by an experimental apparatus consisting of an optical fiber coupled to a portable spectrometer (ocean optics) and a microscope. The excitation source used was a He-Cd laser operating at 325nm. The microstructure emission was collected through the microscope objective and guided to a portable spectrometer by an optical fiber.
As a complementary experiment, macroscopic samples with diameter of about 0.9 cm were prepared by using the same resin composition of the microstructures, cured by UV-lamp and thermally annealed. The generation of nanoparticles in the macroscopic samples was indirectly observed during the heating process by a strong color change in the material, which turned from light yellow to red. The samples were them polished and prepared for absorption spectra measurements. Additionally, transmission electron microscopy TEM images were obtained using a JEOL JEM 2100 URP equipment operating at 200kV. The samples were obtained by milling the cured resin and the produced gold nanoparticles were directly observed in the fragments.
3. Results and discussion
Figure 1 shows a scanning electron microscopy (SEM) image of the two-photon polymerized structures containing gold nanoparticles fabricated using an average power of 25 mW. The microstructures, with dimensions around 20 x 20 µm, present good resolution and low surface roughness, same as presented by the non-doped samples, demonstrating that the thermal treatment doesn´t damage the samples. In addition, measurements of polymerization threshold for the resin mixture with and without HAuCl4 were the same, which indicates that neither the nanoparticles nor the gold acid used in the resin formulation impairs the structure mechanical properties.
Although there are reports on photoreduction of gold ions in a polymer matrix using a similar laser system , laser average power and exposure times are very different. In our setup we used 25 mW incident average power and the laser scanning speed used to fabricate the microstructures is such that each irradiated region of the sample gets exposed for only 20 ms. For this reason we observed no photoreduction of the gold ions embedded in our resin during the polymerization process. The thermal annealing provides the required energy to reduce gold ions to gold atoms and also contributes to increase the atomic mobility in the polymer matrix, favoring the formation of nanoparticles. As a first evidence of the presence of gold nanoparticles in the two-photon polymerized samples, we noted a strong fluorescence that is not observed in the non-heated HAuCl4 doped microstructures. Figure 2(a) and 2(b) show fluorescence microscopy images of HAuCl4 (non-heated) doped and nanoparticle doped microstructures, respectively. Figure 2(c) displays confocal microscopy image of nanoparticle doped microstructures, which shows that the enhanced fluorescence occurs throughout the sample, indicating that the nanoparticles are distributed in the structure bulk.
To investigate the origin of the characteristic emission for microstructures doped with gold nanoparticles, we collected the fluorescence spectra of HAuCl4 (non-heated) doped and nanoparticle doped samples, which are displayed in Fig. 3 , by using as excitation a CW laser operating at 325 nm.
Both samples present a wide fluorescence spectrum, covering almost all the visible range (from 400 nm to 600 nm) although the emission from the gold nanoparticle doped microstructure is considerably more intense. The enhanced fluorescence observed, although very similar to the polymer luminescence, arises from nanoparticles luminescence [30, 31] since excitation wavelength is far from nanoparticles plasmon band. Additional fluorescence measurements (not shown), performed on macroscopic samples using the same experimental apparatus and excitation source, confirm these results. A small blue shift is noticed when one compares the emission from the nanoparticle doped macroscopic sample to the non-doped one, which might be a result of SPR absorption by the gold nanoparticles in the polymer bulk. This effect is not observed in the microscopic sample since its thickness is much smaller, and therefore the absorption of the sample fluorescence is also much smaller, preventing the blue-shift to occur.
To investigate the size and shape of the produced nanoparticles into the polymeric bulk sample, absorption spectra measurements and TEM images of macroscopic samples were obtained. We performed absorption measurements in three different samples, which are displayed as an inset (left side) in Fig. 4 : undoped polymer sample (a); polymer sample with HAuCl4 (b) and polymer sample with thermally generated gold nanoparticles (c). Figure 4 shows the absorption spectra of the three macroscopic samples. The TEM image displayed as an inset of Fig. 4 (right side) confirms the presence of the gold nanoparticles produced in the polymer bulk of sample c, which vary in size (from 5 to 40 nm) and shape. Generally the produced nanoparticles are well dispersed, indicating an in situ production. From TEM images, similar to the one presented in the inset of Fig. 4, we were able to estimate the volume fraction of gold nanoparticles as 4.7%.
The absorption spectrum does not change significantly between samples a) and b) in Fig. 4. However, in sample c) we observe the characteristic plasmon band, centered at 542 nm, with similar results reported in the literature . The conditions for this resonance depend on the size and shape of the nanoparticles and also on the surrounding medium dielectric constant .
We were able to produce two-photon polymerized microstructures doped with gold nanoparticles using a simple and indirect doping method. Our indirect doping method allows the two-photon polymerization fabrication with no interference of the metal nanoparticles in the fabrication process. This feature opens the possibility of fabricating microstructures with more than one dopant, for example, a fluorescent dye and metal nanoparticles to explore the dye fluorescence enhancement properties. Moreover this indirect doping method paves the way to explore other noble metal nanoparticles doped microdevices, which can find applications as a contrast agent for the visualization of 3D structures in confocal microscopy, as well as a tool for sensing chemical and biological substances, in the manufacturing of substrates for SERS measurements and other nanoplasmonics applications that can benefit from the local field enhancement effect. This nanoparticle production method can be applied in many samples, heated simultaneously, opening the possibility for large scale production.
The research described in this paper was supported by FAPESP, CNPq and CAPES from Brazil. Technical assistance from André L.S. Romero is gratefully acknowledged. Authors also would like to thank the Electron Microscopy Laboratory (LME) of the Brazilian National Synchrotron Light Laboratory (LNLS) for the use of the TEM facility.
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