Spatially-targeted laser fabrication of multi-metal microstructures inside a hydrogel

The spatially-targeted fabrication of bimetallic microstructures coexisting in the supporting hydrogel is demonstrated by multi-photon photoreduction. Microstructures composed of gold and silver were fabricated along a predefined trajectory by taking advantages of the hydrogel’s ionic permeability. Different resonant wavelengths of optical absorption were obtained for gold, silver, and their bimetallic structures. Transmission electron microscopy and energy dispersive X-ray analysis revealed that the optical properties are attributable to the formation of bimetallic structure consisted of core–shell nanoparticles. The fabrication of dissimilar metal structures within hydrogel is a promising technique for optically driven actuators in soft robotics and sensing applications by allowing for siteselective optical properties. © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

substrate were fabricated by adhesion lithography for novel applications in flexible devices [11]. Nevertheless, a fabrication technique for bimetallic structures that is applicable to an arbitrary three-dimensional (3D) space would enable various optical applications including optically-driven actuators, photonic crystals, and metamaterials.
Femtosecond laser direct writing via multi-photon processing has emerged as a powerful 3D fabrication technique for micro-and nanostructures. Photopolymerization or photoreduction occur with a high degree of spatial accuracy by multi-photon absorption in the tightly focused space of the laser pulses. Such 3D structures can be fabricated with submicron to multi-micron resolution by adjusting the position of the focal point. Because the fabrication of 3D polymer structures was demonstrated in 1997 [12], many studies have reported on the fabrication of polymer structures based on multi-photon photopolymerization [13][14][15]; beyond this, the fabrication of a multi-polymer structure was demonstrated recently [16]. A few papers have also reported the selective metallization of polymer microstructures that were formed via multi-photon photopolymerization. In 2007, LaFratta et al. fabricated 2D metal microstructures by electroless deposition of copper on the selective-functionalized surface of the acrylic structures [17]. Takeyasu et al. demonstrated the fabrication of 3D silver/polymer conjugated microstructures by site-selective metal deposition on photopolymer structures [18]. Although these methods require multiple complex steps for the fabrication, they can provide site-selective fabrication of metal and polymer multi-structures. The fabrication of 2D [19][20][21] or 3D [22,23] metal microstructures on a glass substrate by multi-photon photoreduction has been demonstrated by focusing femtosecond laser pulses into a metal ion solution or into a polymer material. In these studies, the liquid solution or polymer matrix needed to be removed after fabricating the metal microstructures. Recently, the fabrication of metal microstructures inside a supporting base material containing metal ions was demonstrated [24][25][26]. Metal nanostructures were also fabricated inside glass by laser irradiation of gold-or silver-doped glass substrate [27,28]. With such methods, the fabrication of arbitrary multi-metal structures is difficult because the metal ions must be doped or mixed into a supporting material in advance before the laser irradiation.
Hydrogel, which is a 3D polymer material containing water, is a promising supporting material for metal structures for the realization of novel optical, mechanical, and biomedical devices owing to the good biocompatibility and flexibility [29,30]. Kang et al. reported their pioneering study on the fabrication of silver nanodots in gelatin by multi-photon photoreduction, in which silver ions were provided by mixing in silver nitrate before the cross-linking of the gelatin [26]. Shortly thereafter, we fabricated silver line structures inside poly(ethylene glycol) diacrylate (PEGDA) hydrogel-a typical synthetic polymer hydrogelby multi-photon photoreduction [31,32]. Various applications including optically driven actuators that enable much complex movement as well as wavelength-selective optical sensors would be realized if dissimilar metal structures are able to be fabricated spatial selectively within hydrogel.
In this paper, we demonstrate-for the first time, to the best of our knowledge-the spatially-targeted fabrication of multi-metal structures coexisting in the same supporting hydrogel by multi-photon photoreduction. We took advantage of the hydrogel's permeability to fluids, i.e., we added metal ions alternately by immersing the hydrogels into respective metal ion solutions after photo-cross-linking. The optical absorbance properties of the fabricated bimetallic structures exhibit a tunable wavelength of absorption, which is attributable to the plasmonic resonance of nanostructures as verified by electron microscopy results.

Materials and methods
PEGDA (average molecular mass: 6000) and the photoinitiator Irgacure2959 were purchased from Sigma-Aldrich Co. LLC (St. Louis, MO). PEGDA (0.1 g) was dissolved in 1 ml of pure water containing 1% photoinitiator and stirred for 15 min. The solution was placed in a mold and illuminated by a 365 nm light from a UV lamp (6W) for 20 min to induce photo-crosslinking. The hydrogel of 5 mm in length, 3 mm in width, and 3 mm in thickness was used for the experiments of the optical microscope observation of the fabricated metal microstructures, while the disk-shaped hydrogel 12 mm in diameter and 1 mm in thickness was used for the optical property measurements. The cross-linked hydrogel was stored in pure water overnight. For fabrication of single metal microstructures (i.e., gold or silver), the hydrogel was immersed in gold(III) chloride (Sigma-Aldrich Co. LLC) solution (0.4 mg/ml) or silver nitrate (Sigma-Aldrich Co. LLC) solution (40 mg/ml) for 10 min to add gold or silver ions, respectively; then, femtosecond laser pulses were focused. For fabrication of gold and silver bimetallic microstructures, we used a procedure that took advantage of hydrogel's ionic permeability. Gold structures were fabricated by femtosecond laser irradiation of the hydrogel containing gold(III) chloride solution (0.4 mg/ml); subsequently, the hydrogel was immersed in pure water overnight to remove residual gold ions. The hydrogel was then immersed in silver nitrate solution (40 mg/ml) for 10 min prior to another scanning of femtosecond laser pulses to form the silver structures coexisting with the preformed gold structures in the same hydrogel.
Femtosecond laser pulses with a wavelength of 522 nm at 63 MHz repetition rate, was used for the fabrication of gold and/or silver microstructures by multi-photon photoreduction of the metal ions. The pulse duration of the output pulse was 192 fs. An objective lens (numerical aperture (NA) 0.4, working distance 1.2 mm, Olympus; Tokyo, Japan) was used to focus the laser pulses for the fabrication of the grid formed by gold and/or silver, whereas a water immersion objective lens (NA 1.0, working distance 2.0 mm; Olympus) was used for fabrication when the two gratings crossed at different distances from the surface of the hydrogel. The metal microstructures were fabricated along a predefined trajectory by using a computer-controlled, three-axis encoded (XYZ) motorized stage.
The fabricated structures inside the wet hydrogel were observed with an inverted transmission optical microscope (Eclipse Ti-E, Nikon; Tokyo, Japan). The microscope images of the fabricated structures were obtained by a camera (DS-Ri1, Nikon) attached to the microscope. The hydrogel was dried and thinned using pestle and mortar, and then the thin film of 100 nm thickness was prepared by slicing the sample in an epoxy-molding compound. The slice was observed by transmission electron microscopy (TEM, Tecnai Spirit TEM, FEI; Hillsboro, Oregon). Elemental analysis of the fabricated line structure formed from gold and silver was performed by energy dispersive X-ray (EDX) spectroscopy during scanning TEM (STEM, Tecnai Osiris, FEI). The optical absorbance spectra of the hydrogel in which metal grids or gratings were fabricated were measured using a spectrometer (USB4000, Ocean Optics, Inc.; Largo, Florida). Figure 1 shows a bright-field microscope image of the gold and silver microstructures fabricated in the PEGDA hydrogel, taken 250 μm from the surface of the hydrogel. Three microstructures with different sizes (structure widths of 140 μm, 93 μm, and 47 μm) were fabricated by raster scanning (Fig. 1(c)) according to the predefined model design (Figs. 1(a) and 1(b)). The characters of "Keio Univ" were formed by laser scanning the hydrogel containing gold ions. After the replacement of gold ions to silver ions, the nib (i.e., fountain pen tip)-shaped silver microstructures were fabricated by laser scanning with the presence of silver ions. The result clearly shows dissimilar metal microstructures that were fabricated in the same hydrogel by multi-photon photoreduction. The laser power during the fabrication was 15.0 mW (corresponding to 0.24 nJ per pulse). The scanning speed was 50 μm/s for the fabrication of the structures with widths of 140 μm and 93 μm, whereas the scanning speed was 100 μm/s for the fabrication of the microstructure with a width of 47 μm. Red and yellow colors were observed for the fabricated gold and silver microstructures, respectively, which are attributab result indicate color in the ce Figure 2 s silver. The lin silver structur shows the coe in the same h gray (horizon fabrication of reactions betw ions [33,34]. possibly due Fig. 2(d)). On removal of th fabrication. T [35] between cause the rem face plasmon r bricated micros cters is explaina ield microscop n which the go (Fig. 2(b)) ex old and silver st ever, the silver ig. 2(d)) when e structure. Th and silver met ns reduced by al silver ion w y, the limited from the hydro essure differen and the hydrog ons.

Fabrication of bimetallic structures
ped gold and silv mputer-generated im he fabricated micr characters of "Ke d silver, respective resonances (SP structures com able by the hig e images of lin old line structu xhibit colors r tructures consi r line structure n gold ions w ese results are tal, i.e., the oxi y laser scannin which was redu change of col ogel by water nce, induced by gel containing ver microstructure mage of the prede rostructures inside eio Univ" and th ely. The scale bar r PR) of the m mprise metal na gher density of ne structures fo ures were fabri respective of t isting of their r es changed fro were added to e explainable b idation of silve ng also forme uced simultane lor in Fig. 2

Optical absorbance spectra
The optical absorbance spectra of the fabricated grids and gratings formed from gold and/or silver within hydrogel were obtained ( Fig. 4(a)). The absorbance peaks of the gold and silver gratings were observed at approximately 550 nm and 458 nm, respectively, which are consistent with the typical resonant wavelengths of gold and silver nanoparticles with diameters from 60 to 80 nm [36,37]. However, because the fabricated metal structures were embedded in a hydrogel matrix, it should be noted that the absorbance peaks could be shifted due to the binding to an organic structure [38]. The diameters of the fabricated nanoparticles are assumed to be smaller than 60 nm by considering the shift of the absorbance peak. The absorbance peaks of the gold and silver grids, fabricated by orthogonally-crossed gratings, exhibited red shifts of approximately 15 nm to 18 nm compared to the peaks of the respective metal gratings. The peak wavelengths of metal nanoparticles are well known to depend on the particle's size, showing a red-shift with increasing nanoparticle diameter [37]. The size distribution of the metal nanoparticles could be shifted, probably due to the duplicate scanning of femtosecond laser pulses, which might be significant at the intersection points during grid fabrication. The observed absorbance peak of the gold/silver bimetallic grid was 520 nm. It should be noted that the spectrum does not show double peaks-the typical spectrum for the mixture of two different materials-but a significant single peak that dominates gold and silver peaks. The result may suggest the formation of bimetallic nanostructures such as core-shell or alloy nanoparticles. We attempted to shift the peak wavelength by changing the laser scanning speed of the femtosecond laser during the fabrication of bimetallic grids. The laser scanning speed for the fabrication of gold grating was fixed at 25 μm/s, whereas that for the subsequent silver grating was varied from 5 μm/s to 100 μm/s (Fig. 4(b) and enlarged image of peaks in Fig.  4(c)). The bimetallic grid with silver grating fabricated at 100 μm/s had an absorbance spectrum shifted to longer wavelengths, whereas the bimetallic grid with silver grating fabricated at 5 μm/s had an absorbance spectrum shifted to shorter wavelengths compared to the case at 25 μm/s. Different resonance wavelengths were obtained by controlling the quantity of reduced silver ions by changing the laser scanning speed, i.e., by changing the number of pulses overlapping for the fabrication of silver grating. The peak wavelength would also be controlled much precisely by changing the size of the fabricated nanoparticles, since the peak wavelength depends on the size of nanoparticles [39,40].
Gold and silver gratings crossing at different distances from the surface of the hydrogel were fabricated by changing the focal depth of the femtosecond laser pulses. First, the gold grating was fabricated at 250 μm from the surface of the hydrogel; then, the silver grating was fabricated at various distances closer to the surface. Figure 4(d) shows the absorbance spectra of such grating layers formed from gold and silver; an enlarged image of the peaks is shown in Fig. 4(e). The absorbance peak gradually showed a red-shift as the distance between the two grating layers was increased. The quantity of formed pure gold nanoparticles could have increased, as increasing the distance between two grating layers formed from gold and silver-and hence the resonance wavelength of the two grating layers-shifted the lower wavelength side of the absorbance peak, i.e., the side corresponding to the pure gold nanoparticles' spectrum. The result may also suggest that the quantity of formed bimetallic nanostructures, such as core-shell or alloy nanoparticles, decreased as the distance between two grating layers increased.

Conclusio
We have dem inside the sa fluids. pical morpholo alent diameter gold/silver bim er in Fig. 6(a) h metallic nanopa he cluster in for photo-redu of core-shell e also likely to ated to the abs ction of silver i on to thickenin peak.  (Fig. 5(c)). 5 by EDX ana e structure con r and (c) gold alo d) Schematic illus anoparticle clu for the gold na article cluster w ed nanoparticl in Fig. 6(b). An hich is a typic articles [41]. T noparticles as core and silve ra shown in F ve formed a thi f individual nan in the (a) gold gra 00 nm.  Fig. 6(b)). o be seen rticles are aused by cates-in g. 5-that ggregated wer laser noparticle sulting in ) coexisting ability to scanning nner. The m those of ths of the ging laser and EDX analysis revealed that the fabricated bimetallic structure consisted of core-shell nanoparticles. By allowing for site-selective optical properties, the fabrication of dissimilar metal structures within hydrogel is a promising technique for optically driven actuators. Sensing applications, such as devices for glucose and gas measurements, are also candidates for applying the proposed technique that takes advantage of the permeability of hydrogel.