Multiphoton Photoreduction Research Articles

Surface structuring and in situ synthesis of nanoparticles by ultrashort laser processing on hydroxyapatite

Ultrashort laser pulses represent a versatile tool that finds application in a variety of fields allowing precise and controlled modification of any kind of materials. The material surface modification, by inducing the formation of micro and nanostructures or providing chemical changes, or its functionalization with other materials, in form of nanoparticles, strongly affects the behavior and performances of such materials. The surface structuring and the synthesis and deposition of metallic nanoparticles has both been obtained by laser exposure. The two processes are performed consecutively, on hydroxyapatite sample surface. The synthesis of silver nanoparticles (AgNPs) takes place on the sample surface, while immersed in a solution of silver nitrate (AgNO3), by laser-induced multiphoton photoreduction of silver ions (Ag+).

Formation of Gold Nanoparticles inside a Hydrogel by Multiphoton Photoreduction for Plasmonic Sensing

Formation of Gold Nanoparticles inside a Hydrogel by Multiphoton Photoreduction for Plasmonic Sensing

Light-driven flow control in the hydrogel microchannel by utilizing a metal microstructure fabricated by multiphoton photoreduction

The light-stimulated control of flow velocity in the hydrogel microchannel was demonstrated by utilizing a metal microstructure fabricated by multiphoton photoreduction. The metal microstructure was fabricated adjacent to the microchannel in a poly-N- isopropylacrylamide (PNIPAm) hydrogel. Owing to the photothermal conversion at the metal microstructure, local deformation of the microchannel and change in flow velocity was induced as a result of the volume phase transition of the supporting hydrogel around the metal microstructure. Wavelength-selective change in the flow velocity was realized by utilizing dissimilar metal microstructures, that exhibit different optical resonances. The results indicate that multiphoton photoreduction is a promising method for fabricating novel hydrogel devices having flow-controllable switches by arranging light-absorbing structures with high spatial selectivity.

Single-Step Process for Titanium Surface Micro- and Nano-Structuring and In Situ Silver Nanoparticles Formation by Ultra-Short Laser Patterning

Ultra-short laser (USL)-induced surface structuring combined with nanoparticles synthesis by multiphoton photoreduction represents a novel single-step approach for commercially pure titanium (cp-Ti) surface enhancement. Such a combination leads to the formation of distinct topographical features covered by nanoparticles. The USL processing of cp-Ti in an aqueous solution of silver nitrate (AgNO3) induces the formation of micron-sized spikes surmounted by silver nanoparticles (AgNPs). The proposed approach combines the structuring and oxidation of the Ti surface and the synthesis of AgNPs in a one-step process, without the use of additional chemicals or a complex apparatus. Such a process is easy to implement, versatile and sustainable compared to alternative methodologies capable of obtaining comparable results. Antimicrobial surfaces on medical devices (e.g., surgical tools or implants), for which titanium is widely used, can be realized due to the simultaneous presence of AgNPs and micro/nano-structured surface topography. The processed surfaces were examined by means of a scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM) and Raman spectroscopy. The surface morphology and the oxidation, quality and quantity of AgNPs were analyzed in relation to process parameters (laser scanning speed and AgNO3 concentration), as well as the effect of AgNPs on the Raman signal of Titanium oxide.

Role of the Polymeric Matrix and Surface Affinity During Gold Nanoparticles Patterning by Multi-Photon Photo-Reduction

Gold nanoparticles (GNPs) can be patterned on specific positions and substrates by Multi-Photon Direct Laser Writing (MP-DLW) in wet polymeric matrices doped with tetrachloroauric acid (HAuCl₄) as gold precursor. The Hamaker constants describing the GNP-GNP interaction and the interaction between the GNPs and the surrounding materials is described, defining the role of the polymeric matrix; thus, its limits and the advantages are thoroughly analysed. The Multi-Photon Photo-Reduction (MPPR) process, leading to the GNPs creation, triggers a local temperature rising, which can ablate the polymer and influences the particle distribution, size and density. The GNPs polydispersity also depends on the water content in the film, variable because of the vaporization. A protocol to perform MP-DLW of GNPs, without the use of the polymer is illustrated: by treating the surface with a surfactant, it is possible to make the particles stick to the substrate, control their size and reduce the diffusive and convective effects generated by the MPPR.

Plasmon-enhanced nanosoldering of silver nanoparticles for high-conductive nanowires electrodes

The silver nanowires (Ag NWs) electrodes, which consist of incompact Ag nanoparticles (NPs) formed by multi-photon photoreduction, usually have poor conductivities. An effective strategy for enhancing conductivity of the Ag NWs electrodes is plasmon-enhanced nanosoldering (PLNS) by laser irradiation. Here, plasmon-enhanced photothermal effect is used to locally solder Ag NPs and then aggregates of these NPs grow into large irregular particles in PLNS process. Finite element method (FEM) simulations indicate that the soldering process is triggered by localized surface plasmon-induced electric field enhancement at “hot-spots”. The effectiveness of PLNS for enhancing conductivity depends on laser power density and irradiation time. By optimizing the conditions of PLNS, the electrical conductivity of Ag NWs is significantly enhanced and the conductivity <i>σ</i>_s is increased to 2.45×10⁷ S/m, which is about 39% of the bulk Ag. This PLNS of Ag NWs provides an efficient and cost-effective technique to rapidly produce large-area metal nanowire electrodes and capacitors with high conductivity, excellent uniformity, and good flexibility.

Additive Manufacturing of Gold Nanostructures Using Nonlinear Photoreduction under Controlled Ionic Diffusion.

Multiphoton photoreduction of photosensitive metallic precursors via direct laser writing (DLW) is a promising technique for the synthesis of metallic structures onto solid substrates at the sub-micron scale. DLW triggered by a two photon absorption process is done using a femtosecond NIR laser (λ = 780 nm), tetrachloroauric acid (HAuCl4) as a gold precursor, and isinglass as a natural hydrogel matrix. The presence of a polymeric, transparent matrix avoids unwanted diffusive processes acting as a network for the metallic nanoparticles. After the writing process, a bath in deionized water removes the gold precursor ions and eliminates the polymer matrix. Different aspects underlying the growth of the gold nanostructures (AuNSs) are here investigated to achieve full control on the size and density of the AuNSs. Writing parameters (laser power, exposure time, and scanning speed) are optimized to control the patterns and the AuNSs size. The influence of a second bath containing Au3+ to further control the size and density of the AuNSs is also investigated, observing that these AuNSs are composed of individual gold nanoparticles (AuNPs) that grow individually. A fine-tuning of these parameters leads to an important improvement of the created structures’ quality, with a fine control on size and density of AuNSs.

Plasmon-enhanced nanosoldering of silver nanoparticles for high-conductive nanowires electrodes

The silver nanowires (Ag NWs) electrodes, which consist of incompact Ag nanoparticles (NPs) formed by multi-photon photoreduction, usually have poor conductivities. An effective strategy for enhancing conductivity of the Ag NWs electrodes is plasmon-enhanced nanosoldering (PLNS) by laser irradiation. Here, plasmon-enhanced photothermal effect is used to locally solder Ag NPs and then aggregates of these NPs grow into large irregular particles in PLNS process. Finite element method (FEM) simulations indicate that the soldering process is triggered by localized surface plasmon-induced electric field enhancement at “hot-spots”. The effectiveness of PLNS for enhancing conductivity depends on laser power density and irradiation time. By optimizing the conditions of PLNS, the electrical conductivity of Ag NWs is significantly enhanced and the conductivity <em>σ</em>_s is increased to 2.45×10⁷ S/m, which is about 39% of the bulk Ag. This PLNS of Ag NWs provides an efficient and cost-effective technique to rapidly produce large-area metal nanowire electrodes and capacitors with high conductivity, excellent uniformity, and good flexibility.

Anionic fluorophore-assisted fabrication of gold microstructures inside a hydrogel by multi-photon photoreduction

The fabrication of accentuated gold microstructures is demonstrated by multi-photon photoreduction inside an anionic fluorophore-containing hydrogel. We attempted to facilitate gold-ion photoreduction near the focal point of laser pulses, expecting the donation of electrons by the oxidation of fluorophores in the vicinity of gold ions. The presence of anionic FITC-dextran also inhibited the spontaneous reduction in untargeted zones, which is attributed to the coordination of gold ions and the anionic FITC-dextran. Simultaneous facilitation and inhibition are promising for the fabrication of dense metal microstructures in the targeted zone while maintaining the hydrogel’s light permeability.

Insight into diffusive and convective processes affecting gold nanoparticles microclustering by multiphoton photoreduction

Multi-photon direct laser writing (MP-DLW) in polymeric matrices doped with a tetrachloroauric acid (HAuCl4) water solution allows creating clusters of gold nanoparticles (GNPs) inside the focus figure of a tightly focused ultrafast laser beam. The key physical phenomena involved in the process are analyzed, with the aim to assess the limits and potential of this promising technology. Multi Photon Absorption (MPA) triggers the photo-reduction of AuCl4– ions and the consequent creation of GNPs in the spotted volume. Thermal electronic decays lead to a local abrupt increase of the temperature, which influences the morphology of the created structures. At the same time, two different effects take place, related to the dehydration of the polymeric matrix, and the concentration gradient of the gold precursor upon localized photoreduction. Given their different timescales, these phenomena allow for controlling the GNPs density and size dispersity when a given energy dose is delivered in multiple exposures, tuning the delay between consecutive laser exposures. A simple yet effective experiment to estimate the temperature distribution at the micron-scale is also proposed.

Multiphoton photoreduction for dual-wavelength-light-driven shrinkage and actuation in hydrogel

The dual-wavelength-driven shrinkage of metal microstructures and hydrogel actuation are demonstrated by the fabrication of multi-metal microstructures in hydrogels by multiphoton photoreduction. Silver and gold microstructures were fabricated in a poly-N-isopropylacrylamide (PNIPAm) hydrogel. Because of the different optical resonances of the metals, wavelength-dependent shrinkage of metal microstructures was demonstrated concurrently with the volume change of the supporting hydrogel by light stimulation. Furthermore, the direction of actuation of the hydrogel was controlled by switching the wavelength of light stimulation. The results indicate the potential of multiphoton photoreduction for applications in light-driven optical components and micro-robots fabricated with soft materials.

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 site-selective optical properties.

Shrinkable silver diffraction grating fabricated inside a hydrogel using 522-nm femtosecond laser

The integration of metal microstructures and soft materials is promising for the realization of novel optical and biomedical devices owing to the flexibility and biocompatibility of the latter. Nevertheless, the fabrication of three-dimensional metal structures within a soft material is still challenging. In this study, we demonstrate the fabrication of a silver diffraction grating inside a biocompatible poly(ethylene glycol) diacrylate (PEGDA) hydrogel by using a 522-nm femtosecond laser via multi-photon photoreduction of silver ions. The optical diffraction pattern obtained with the grating showed equally spaced diffraction spots, which indicated that a regular, periodic silver grating was formed. Notably, the distance between the diffraction spots changed when the water content in the hydrogel was reduced. The grating period decreased when the hydrogel shrank owing to the loss of water, but the straight shapes of the line structures were preserved, which demonstrated the optical tunability of the fabricated structure. Our results demonstrate the potential of the femtosecond laser-based photoreduction technique for the fabrication of novel tunable optical devices as well as highly precise structures.

Two-beam laser fabrication technique and the application for fabricating conductive silver nanowire on flexible substrate

In this study, a two-beam laser fabrication technique is proposed to fabricate silver nanowire (AgNW) on the polyethylene terephthalate (PET) substrate. The femtosecond pulse laser in the technique plays a role in generating Ag nanoparticles from the silver aqueous solution by multiphoton photoreduction. The continuous wave (CW) laser of the technique works as optical tweezers, and make the Ag nanoparticles gather to a continuous AgNW by the optical trapping force. The optical trapping force of the CW laser was calculated under our experimental condition. The flexibility and the resistance stability of the AgNW that fabricated by this technique are very excellent. Compared to the resistance of the AgNW without bending, the decreasing rate of the AgNW resistance is about 16% under compressed bending condition at the radius of 1 mm, and the increasing rate of the AgNW resistance is only 1.3% after the AgNW bended about 3500 times at the bending radius of 1 mm. The study indicates that the AgNW is promising for achieving flexible device and would promote the development of the flexible electronics.

Laser Direct Writing of Silver Nanowire with Amino Acids-Assisted Multiphoton Photoreduction

We have demonstrated an approach of silver nanostructure microfabrication by femtosecond laser direct writing of silver ion aqueous solution containing biocompatible amino acids with different molecular architectures. The amino acids-assisted multiphoton photoreduction (MPR) lowers the incident femtosecond laser threshold to 0.32 mW for glycine, 0.72 mW for valine, 0.6 mW for proline, and 0.35 mW for leucine. The line widths of the silver nanowires are varied with the composition of the biocompatible amino acids. The highest resolution of 186 nm has been achieved. Furthermore, the corresponding electrical conductivities of silver nanowires are determined to be 1.48 × 10–6, 4.1 × 10–7, 7.04 × 10–7, and 1.35 × 10–6 Ω m, which have been significantly improved compared to that of the previously reported silver wires fabricated by MPR microfabrication. The proposed protocol for fabricating silver nanostructure would pave the way for fabricating metallic nanostructures and play an extremely important role in th...

Femtosecond direct laser writing of gold nanostructures by ionic liquid assisted multiphoton photoreduction

Amino-terminated ionic liquid assisted multiphoton photoreduction (IL-MPR) was developed for the direct writing of subwavelength gold nanostructures in AuCl4- ions aqueous solution by femtosecond laser. It was revealed that the carbon chain length was crucial for morphology and size control of gold nanostructures. A 228 nm width of gold nanostructure, which was beyond the optical diffraction limit, was fabricated by the matching between IL and the power and scanning speed of the laser beam. The measured conductivity is of the same order as that of bulk gold. Furthermore, we successfully fabricated a U-shaped terahertz planar metamaterial whose spectral response is consistent with the theoretical expectation. The IL-MPR nanofabrication protocol is expected to play an important role in the fabrication of fine metallic micro/nanostructures for applications in microelectromechanical systems, nanoelectronics and nanophotonics.

Gold nanoparticles prepared by glycinate ionic liquid assisted multi-photon photoreduction

We have successfully prepared gold nanoparticles (AuNPs) with flower-like and spherical morphology through multi-photon photoreduction (MPR) of an aqueous solution of HAuCl(4) and (2-hydroxyethyl) trimethylammonium glycinate ([HETMA][Gly]) ionic liquid (IL) through the use of a femtosecond laser. The results of (1)H NMR and UV-Vis absorption indicated that AuNPs were produced from the photoreduction of the [Gly]-Au(iii) complex. Spherical AuNPs of about 2.5 nm were obtained on the solution when irradiated for 2 h, then aggregated into flower-like AuNPs of several tens of nanometers assisted by the IL with an increase in the irradiation time. Furthermore, precipitates of spherical AuNPs with the size of around 15 nm were formed after being irradiated for 6 h. The mechanisms of the MPR reaction and controlled growth of AuNPs have also been discussed.

Morphology Modification of Silver Microstructures Fabricated by Multiphoton Photoreduction

We have investigated the morphology modification of silver microstructures fabricated by the multiphoton photoreduction process. The microstructures have been fabricated by a femtosecond laser under different irradiation time and repeated scanning numbers. Trans-4-[4-(dimethylamino)-N-methylstilbazolium] p-tosylate (DAST) was used as photosensitizer and effectively reduced the laser power to 0.66 mW. The increase of the irradiation time and repeated scanning induced more reduction in the multiphoton photoreduction microfabrication process, resulting in the optimization of the linewidth. The fusion of silver nanoparticles was confirmed, which led to the morphology change of silver microstructures for achieving the compact metallic microstructures. The result would provide an important protocol to fabricate the metallic microstructures for the electronic and photonic applications.

Morphology and size dependence of silver microstructures in fatty salts-assisted multiphoton photoreduction microfabrication

The morphology and size dependence of silver microstructures in a novel microfabrication process, fatty salts-assisted multiphoton photoreduction (MPR), were investigated by using the fatty salts with different carbon chain lengths (Cn: n=4,5,7,9) under varied powers and irradiation times of a femtosecond near-infrared laser with the wavelength of 800 nm. Not only the feature size of the silver structures was reduced but also the surface smoothness was improved by increasing the chain length of the fatty salts. The highest resolution of a silver line was obtained to be 285 nm, which exceeded the diffraction limit. The fatty salts-assisted MPR microfabrication approach would provide an efficient protocol for fabricating metallic micro/nanostructures with fine morphology and size and could play an important role in the fabrication of the metallic micro/nanostructures for applications in photonics and electronics as well as in sensors.