In this study, three-dimensional (3D) polyacrylamide microstructures containing gold nanorods (AuNRs) were fabricated by two-photon polymerization (TPP) using Rose Bengal (RB) as the photoinitiator. To retain AuNRs in the 3D polymer microstructures, the laser wavelength was chosen for two-photon RB absorption for improved TPP efficiency, but not for enhancing the longitudinal plasmon resonance of AuNRs which may result in photothermal damage of AuNRs. After TPP processing, the laser wavelength was tuned for the longitudinal plasmon resonance and the laser power was increased to beyond the damage threshold of the AuNRs for reshaping the AuNRs into gold nanospheres. As a result, AuNRs in designated positions of the fabricated 3D microstructures can be achieved. Two-photon luminescence from the doped AuNRs can also act as contrast agent for the visualization of 3D polymer microstructures.
©2010 Optical Society of America
Photopolymerization is a process which uses a combination of light with low molecular weight photoinitiators to trigger the polymerization reaction [1–3]. To fabricate three-dimensional (3D) microstructures, multiphoton excited (MPE) photochemistry can be used. Since multiphoton absorption is confined to the focal volume, photopolymerized structures with the desired 3D submicron features can be created [4–8]. This approach not only allows the creation of structures that cannot be constructed by conventional single-photon lithography, but also provides greater spatial resolution than other 3D microfabrication techniques. Therefore, multiphoton photopolymerization has attracted widespread interest owing to its potential use in fabricating intrinsic 3D microstructures with sub-diffraction limited spatial resolution . The utilization of short pulse width and tight focusing are critical for inducing sufficient extent of two-photon absorption (TPA) and for achieving high precision fabrication. For the work presented hereafter, a femtosecond titanium-sapphire (ti-sa) laser was as the excitation source. Previously, femtosecond 3D microfabrication has been demonstrated in resin- [3,4,9,10], protein- , silica- , and metal-substrates [12,13]. In additional, photonic crystals have also been demonstrated to be photopolymerizable by TPA [14–16].
Among the developed nanomaterials for biomedical applications, gold nanoparticles (AuNPs) are particularly attractive due to their biocompatibility. Since biomedical applications often involve 3D specimen manipulation, near infrared (NIR) light is preferred due to its optimal tissue transmission from reduced scattering and energy absorption. As a result, maximum irradiation penetration through tissue and minimization of the auto-fluorescence of non-target tissue can be achieved . Therefore, there have been efforts aiming at shifting the surface plasmon resonance of AuNPs into the NIR region for potential biological applications [18,19]. Numerous NIR-absorbing, Au-based bionanomaterials have been developed. Specifically, Au nanorods (AuNRs) with different aspect ratios and corresponding longitudinal plasmon resonance in the 700 to 1000 nm fall within the spectral range of the femtosecond ti-sa laser. AuNRs designed with these properties have been applied in localized surface plasmon resonance light scattering , Rayleigh elastic scattering , surface-enhanced Raman inelastic scattering , optical coherent tomography scattering , two-photon luminescence (TPL) imaging , and photothermal therapy [17–19].
While AuNRs have found wide biomedical applications, the use of AuNRs in microfabrication has been rare. Due to the fact that metallic nanoparticles-polymer composites exhibit unique electrical, optical, and mechanical properties, in this work, we explored these properties for microfabrication. Specifically, multiphoton photopolymerization using acrylamide as a reactive monomer, Rose Bengal (RB) as a photoinitiator, triethanolamine (TEA) as co-initiator, dimethyl sulfoxide (DMSO) as surfactant, and AuNRs were used to direct the 3D assembly of polymer microstructures. We present the first example of 3D polymer microstructures containing two-photon excitable AuNRs from the minimally-damped surface plasmon resonance effects [24–26]. We improved the fabrication efficiency at a threshold laser power without damage to the AuNRs by mixing a suitable content with acrylamide, RB, TEA, DMSO, and AuNRs. Moreover, an optimal laser wavelength was chosen for RB TPA, but not for AuNR absorption. Furthermore, the selectivity of AuNRs with different aspect-ratios in different locations can be implemented by photothermal reshaping . Herein, a higher laser power, greater than the threshold of the AuNR damage at the wavelength for the AuNR absorption, was adopted to reshape the AuNRs at the designated positions. The experimental results demonstrate that the developed 3D polymer microstructures containing the AuNRs not only improves efficiency by decreasing the power density of the femtosecond laser, but also provides a great diversity of optical properties that can act as attractive contrast agents for TPL imaging in 3D fabricated microstructures. We will also demonstrate that the TPL from doped AuNRs can be used for internal diagnosis of 3D polymer microstructures.
2. Sample preparation and microfabrication setup
2.1. Sample preparation
Hydrogen tetrachloroaurate (III) hydrate (HAuCl4) was purchased from Alfa Aesar Co. (Ward Hill, MA, USA). Cetyltrimethylammonium bromide (CTAB), ascorbic acid, sliver nitrate, sodium tetrahydrdoborate, acrylamide, bis-acrylamide, RB, TEA, and DMSO were purchased from Sigma-Aldrich Co. (St. Louis, MO, USA). All chemicals and reagents were of the analytical grade. AuNRs were synthesized using the seedless growth method [17–19,28,29]. The mean length and width of the AuNRs were approximately 40.2 nm and 11.2 nm, respectively, and their aspect ratio (length divided by width) was about 3.6. A resin, acrylamide/bis-acrylamide (74:1), was utilized as a reactive monomer. The fabrication solution is consisted of 2.0 mM RB solution (photoinitiator), 0.1 M TEA solution (co-initiator), and 10% (v/v) DMSO (surfactant) mixed into the monomer solution.
Electron micrographs of the AuNRs using transmission electron microscopes (TEM) (Jeol 1200, at 80 kV; Jeol 3010, at 300 kV; and Philips CM-200, at 200 kV, Japan) were obtained by placing a drop of the sample on a copper mesh coated with an amorphous carbon film. After evaporation of the solvent was in a vacuum desiccator, the samples were placed on ITO-coated glass slides for the specimen characterization using a scanning electron microscope (SEM) (Jeol 7000). The UV/Vis absorption spectra were recorded on a spectrometer (Agilent 8453, USA), while the Fourier transform infrared (FTIR) spectra were collected using another spectrometer (PerkinElmer RX1, USA). Finally, the data of zeta potential were measured using a spectrometer (Manern Nano-ZS90, UK).
AuNRs with a CTAB surfactant coating were synthesized using the seedless growth method. Due to the presence of CTAB, the surface charge of the AuNRs revealed a zeta potential of approximately 39.2 mV. As shown in Fig. 1(a) , AuNRs exhibit two plasmon resonances with a transverse plasmon (about 520 nm in water) and a longitudinal plasmon (about 760 nm in water; 780 nm in the fabrication solution and the polymer). Also shown in Fig. 1(a) is the TEM image of the AuNRs demonstrating that these materials have an aspect ratio of approximately 3.6 (length: 40.2 nm, width: 11.2 nm). On the other hand, the absorption spectrum of RB exhibited one main peak at around 550 nm (Fig. 1(b)), while a similar band appeared as the RB was added to the fabrication solution with the AuNRs (Fig. 1(a)). However, UV/Vis spectra results in Fig. 1(a) show that there were no apparent differences in the longitudinal plasmon of the AuNRs despite decrease in the RB absorption after the polymerization of acrylamide. Shown in Fig. 1(b) are the absorption spectra of acrylamide/bis-acrylamide, aqueous RB, TEA, and DMSO (concentrations of the RB in Figs. 1(a) and 1(b) are both 2.0 mM). Since the spectra in Fig. 1(a) including the transverse plasmon effect of the AuNRs, the absorbance at around 550 nm in Fig. 1(a) is higher than that in Fig. 1(b).
2.2. Femtosecond laser microfabrication system
Shown in Fig. 2 is the schematic of the femtosecond laser fabrication system. Key components of our instrument include a femtosecond laser (Tsunami, Spectra-Physics, USA), an inverted optical microscope (Axiovert 200, Zeiss, Germany), galvanometer x-y scanner (6215H, Cambridge, USA), a triple-axis sample positioning stage (ProScanTMII, Prior, UK), a z-axis piezoelectric nano-positioning stage (Nano-F100, Mad City Labs, USA), an acousto-optic modulator (AOM) (23080-x-1.06-LTD, Neos, USA), photomultiplier tubes (PMTs) (H5783P, Hamamatsu, Japan), and a data acquisition (DAQ) card with a field-programmable gate array (FPGA) module (PCI-7831R, National Instruments, USA). The femtosecond ti-sa laser source has a pulse width of less than 100 fs and a repetition rate of 80 MHz. The FPGA module was designed to perform a number of simultaneous tasks including control of the galvanometer scanner and the z-axis piezoelectric stage for 3D focal spot positioning; modulating the AOM for rapid on/off switching of the laser and pulse selection; and processing of the single photon counting (SPC) signals. Selected experimental parameters such as laser power, scanning rate, imaging, and sample positioning can be adjusted by the use of custom LabVIEW program and electronics interfaces. In this manner, imaging with nonlinear optical signals (two-photon fluorescence (TPF)/second harmonic generation (SHG)) and 3D microfabrication can be achieved. To overcome the group velocity dispersion of the femtosecond laser through the AOM and the objective, an SF-10 prism pair (PC-TS-KT, Newport, USA) was used for optimized laser operation in the wavelength region between 700 to 840 nm .
The real-time FPGA DAQ card based on our custom LabVIEW program can synchronously control the instrument through interfaces constructed in-house. To achieve a fast and precise scan, the digital and analog I/O signal from the programming FPGA was utilized. A digital I/O via a voltage converter is connected into the AOM for fast on/off laser control, with a speed of up to 9 MHz. The voltage converter reduces the 3.3 V command signal from the FPGA digital input/output to below 1.0 V to match the requirement of the AOM driver. The SPC-based pulse counting scheme is based on the FPGA digital input/output. Specifically, the number of high to low electronic transitions signals from a discriminator-processed PMT was determined. The pulse counter records one count when the voltage level underwent a high to low transition. The highest operating frequency of our FPGA module is 200 MHz, which is sufficient for determining the processing signals associated with our 80 MHz femtosecond laser source.
2.3. Designing 3D freeform structures
In addition to nonlinear optical imaging capabilities, CAD software such as AutoCAD, Pro/E, and Solidworks can be used to design 3D structures for microfabrication. To transform 3D structures into 2D processing patterns, transformation programs such as Rhino and Materialse Magics can be adopted to convert the 3D structures into sequential 2D DXF files . The 2D DXF files are then converted into bitmap files and downloaded into the FPGA module as laser processing commands. With the use of 3D structure design, we were able to create the desired structures.
3. Experimental results and discussions
With the fabrication solution consisting of acrylamide/bis-acrylamide, RB, TEA, DMSO, and AuNRs, we were able to improve the polymerization efficiency. More than 1.0 mM of the RB was required to provide adequate photoinitiation processing, while the DMSO surfactant is for the uniform dissolution of TEA. The AuNRs with sufficient CTAB can be dissolved in the fabrication solution. In order to implement the multiphoton fabrication of 3D polymer microstructures without femtosecond laser damage to the AuNRs, it is important to adopt a fabrication laser power as low as possible with a wavelength appropriate for the RB TPA, but not for the AuNR absorption. After multiphoton fabrication, the selectivity of AuNRs with different aspect-ratios in different locations can be achieved by photothermal reshaping. Moreover, a higher laser power, greater than the threshold of the AuNR damage and at the resonance wavelength of their longitudinal plasmon, was utilized to reshape the AuNRs into Au nanospheres. As a result, the existence of the AuNRs in designated positions of the fabricated microstructures can be achieved.
3.1. Wavelength selection in femtosecond laser microfabrication
In TPP processing, we can improve the polymerization efficiency of the acrylamide/bis-acrylamide monomer by adopting the laser wavelength at the maximum TPA of the photoinitiator, RB. Upon excitation, the time-averaged TPF photon count (F) of a fluorescence species is proportional to the cross section (δ) of TPA and can be given as 
In TPA spectrum measurement experiment, the TPF photon counts were collected by the PMTs via the SPC module at the x galvanometer scanner rate of 20 kHz and the average excitation power of 10.0 mW. The excitation spectral range was selected from 710 to 830 nm and the pulse widths at different wavelengths after the objective were monitored by an in-lab constructed autocorrelator. The relative TPA spectrum of the RB (2.0 mM) in DI water as a function of the excitation wavelength is shown in Fig. 3 . It was found that within the excitation wavelengths we examined, the excitation wavelength corresponding to the maximum value of the relative TPA of the RB was between 710 and 720 nm. Therefore, a fabrication laser wavelength of around 720 nm was adopted. Moreover, in order to implement the multiphoton fabrication of 3D polymer microstructures with AuNRs, the wavelengths of the two plasmon resonances of the adopted AuNRs should differ significantly from the fabrication wavelength of 720 nm. As shown in Fig. 1(a), the AuNRs with an aspect ratio of approximately 3.6 exhibits two plasmon resonances with transverse plasmon at around 520 nm and longitudinal plasmon at 780 nm in the fabrication solution. However, the AuNRs with a longitudinal plasmon wavelength longer than 780 nm are also good candidates for the 3D multiphoton fabrication process with the RB.
3.2. Selective AuNR reshaping by femtosecond laser
In order to implement multiphoton fabrication of 3D polymer microstructures, the power of the 100 fs femtosecond laser at the repetition rate of 80 MHz must be sufficient to support the MPE photochemistry process. According to our experience, the use of NA 1.3 objective and the x-galvanometer scan rate of 1 kHz, the laser power at the TPA wavelength of the photoinitiator must be controlled to within at least a few mW to implement the multiphoton fabrication in our solution. Furthermore, since AuNRs can be easily reshaped by utilizing the 100 fs laser at the resonance wavelength of the longitudinal plasmon of the AuNRs, the threshold power for completely melting AuNRs within a linearly polarized (along the longitudinal axis of the AuNRs) laser pulse was about 0.96 mW . However, from the simulation results based on a two-temperature model, the threshold power for the 100 fs linearly polarized laser at the resonance wavelength is 0.049 mW . When the polarization of the laser is turned from linear to circular, the threshold power can be decreased half. Also, the orientation-specific damage for the long axis of AuNRs parallel to the direction of the linearly polarized laser can be avoided. From our results, the threshold power for AuNR melting using the circularly polarized, femtosecond laser at the resonance wavelength was 0.5 mW. This laser power would result in the minimal damage of AuNR for all of the orientations. Based on the AuNR absorption spectra in Fig. 1(a), the minimum AuNR damage power at the RB TPA wavelength of around 720 nm can be several times higher than 0.5 mW, the power used in processing the AuNRs at the longitudinal plasmon resonance wavelength of 780 nm.
In our experiments, a quarter wave plate (QWP) was inserted after the linear polarizer (Fig. 2). The fabrication laser power of 1.0 mW at the optimal fabrication wavelength of 720 nm was used (RB concentration 2.0 mM) . Due to the generation of highly efficient TPL, the AuNRs can be attractive contrast agents for imaging 3D fabricated microstructures. Therefore, we examined the tomographic profile of the AuNRs filled fabricated microstructure by TPL imaging. Figure 4(a) shows the TPL image of a fabricated 10 x 10 μm2 square polymer microstructure filled with AuNRs. In this study, TPL images were excited by the use of 0.1 mW, 100 fs laser at 780 nm and a scan rate of 20 kHz. The SEM zoom in image shows clear and intact AuNRs inside the polyacrylamide (Fig. 4(b)). These results indicate that there was no change to the morphology of the AuNRs after the femtosecond laser fabrication process, even when 1.0 mW fabrication laser power was used. It is likely that most of the laser energy was dissipated into the RB for photopolymerization, not for AuNR absorption.
The 780 nm AuNRs with different orientations in the 3D polymer microstructures can be selectively processed in designated positions via the photothermal reshaping mechanism. For this purpose, the ti-sa wavelength was tuned to 780 nm, the longitudinal plasmon resonance of the AuNRs, and circularly polarized laser was adopted to reshape the AuNRs in all of the orientations. To completely reshape the AuNRs, the laser power was increased to 5.0 mW. Figure 5(a) shows the cross TPL image of the fabricated AuNR-doped microstructure (Fig. 4(a)) after photothermal reshaping to destroy the AuNRs outside of the cross pattern. The TPL image was acquired under the same imaging condition as in Fig. 4(a). The SEM zoom in image indicates that the AuNRs were completely reshaped into spherical AuNPs at 5.0 mW (Fig. 5(b)).
3.3. 3D microfabrication
For 3D microfabrication, the sequential 2D bitmap files sliced from a 3D CAD model were downloaded into the FPGA to control the laser illumination via the AOM. Since TPP is confined to the focal volume, 3D freeform polymer solid structures can be developed. Unreacted solution was then washed out by water. Figure 6(a) shows the TPL image of a hollow 3D microstructure by utilizing the fabrication and imaging conditions described in Sec. 3.2. The structure has a base area of 20 x 20 μm2 and a height of 5 μm. Finally, the diameter of the hole was 10 μm and the distance between two adjacent axial layers was 0.1 μm. Figure 6(b) is the cross-sectional image of the base of the microfabricated structure of Fig. 6(a). The fabrication spatial resolution of the 3D polyacrylamide microstructure we achieved is not as fine as ethoxylated trimethylolpropane triacrylate polymerization microstructures . However, the water soluble acrylamide monomers were uniformly mixed with AuNR. Moreover, the polyacrylamide material is biocompatible, and therefore ready for bioapplications.
The biocompatible 3D microstructure of polyacrylamide with AuNRs was fabricated via the TPP processing. To avoid AuNRs damage in the 3D microstructures, the fabrication laser wavelength (at 1.0 mW) was tuned for RB TPA, not for longitudinal plasmon resonance. Furthermore, a 5.0 mW laser power at the wavelength for the longitudinal plasmon resonance was adopted to reshape the AuNRs into AuNPs in designated positions within the 3D microstructures. Furthermore, doped AuNRs within TPL act as a contrast agent for internal visualization of the fabricated 3D microstructures. The approach described in this work can be used to create microstructures for biomedical applications.
This work was supported by the National Research Program for Genomic Medicine (NRPGM) of the National Science Council (NSC) in Taiwan (NSC 97-3112-B-006-013), (NSC 97-3111-B-006-004), and Advanced Optoelectronic Technology Center of National Cheng Kung University.
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