Materials
SiO 2 colloidal particles with various sizes (140 nm to 5 μm, 5 wt%), green-fluorescent SiO 2 (1 μm, excitation/emission: 497/530 nm, 2.5 wt%) and red-fluorescent polystyrene nanospheres (500 nm and 2.5 μm, excitation/emission: 530/607 nm, 2.5 wt%) were purchased from microParticles GmbH. Mineral Oil Rotational Viscosity Standard (494.0 mPa s), oleic acid (technical grade, 90%), silicone oil AR20 (viscosity: approximately 20 mPa s), CTAB (98%), SDS (98.5%), PEG (Mn: 400), PF108 (Mn: 14,600), TiO 2 NWs (powder, 100 nm × 10 μm), WO 3 NWs (powder, 50 nm × 10 μm), Al 2 O 3 NWs (powder, 2–6 nm × 200–400 nm), CdTe quantum dots (powder, COOH functionalized, fluorescent emission: 710 nm), iron oxide (Fe 3 O 4 ) powder (50–100 nm, 97%), Fe 3 O 4 (20 nm, 5 mg ml−1, dispersed in H 2 O), TiO 2 nanoparticles (150 nm, 900 nm, 5 wt%, dispersed in H 2 O), Au urchin nanoparticles (50 nm, in 0.1 mM PBS), Pt nanoparticles (powder, 50 nm, 99.9%), silver (Ag) powder (<100 nm, PVP as dispersant, 99.5%), orange fluorescent PLGA nanospheres (excitation/emission: 530/582 nm, 100 nm), diamond nanopowder (<10 nm, 97%), trichloro(1H,1H,2H,2H-perfluorooctyl) silane (97%), sodium chloride (NaCl, 99%), H 2 O 2 (30% w/w in H 2 O) and propylene glycol monomethyl ether acetate (PGMEA, 99.5%) were purchased from Sigma-Aldrich. Isopropyl alcohol (IPA, 99.9%) was purchased from Carl Roth GmbH. IPS photoresist was purchased from Nanoscribe GmbH. Immersion oil 518 F (viscosity of about 500 mPa s) was purchased from Fisher Scientific.
Fabrication of 3D hollow microtemplates
The 3D hollow polymeric templates were designed using SOLIDWORKS 2021 and fabricated by direct laser writing using a commercial Photonic Professional GT system (Nanoscribe GmbH). The templates were printed with commercial IPS photoresist in oil-immersion mode with a 63× objective and printed on a fused glass substrate that had been treated overnight with trichloro(1H,1H,2H,2H-perfluorooctyl) silane vapour. The laser power and printing speed were 50 mW and 50,000 μm s−1, respectively. After printing, the structures were developed in IPA solution for 5 min to remove non-polymerized photoresist. The templates were then directly used for the particle assembly in particle-laden dispersions. For aqueous dispersion, the high thermal conductivity of water can limit photothermal efficiency. To enhance this, a 5-nm Cr layer followed by a 5-nm Au layer was sputtered onto the substrate, improving photothermal performance.
Process of the optofluidic 3D microfabrication/nanofabrication
SiO 2 particles of different diameters (150 nm and 1 μm) were used in the assembly experiments. To redisperse SiO 2 particles into various solvent systems, 100 μl of as-received aqueous SiO 2 solution was first centrifuged at 8,000 rpm for 3 min. The supernatant was discarded and the collected SiO 2 precipitate was dried by heating at 60 °C for 2 h. The dried SiO 2 powder was then redispersed in 1 ml of various solvents (for example, immersion oil, oleic acid) and sonicated for 1 h to ensure thorough dispersion. These hydrophilic SiO 2 particles can remain dispersed in various oils in the experimental timescale owing to the high viscosity of the medium, which helps prevent clogging the template openings by large agglomerations during assembly (Supplementary Fig. 15). The resulting SiO 2 suspension was used for the assembly experiments. It should be noted that other colloidal materials were also redispersed in appropriate solvents using a similar procedure to perform the assembly experiments. Furthermore, co-assembly of different materials can be obtained by preparing a mixed suspension through physical blending of various particles, as demonstrated in Fig. 4i, in which SiO 2 microspheres with 600 nm and 1 μm diameter were co-assembled into a single structure. The detailed parameters of colloidal materials used in this study are summarized in Supplementary Table 3.
For experiments conducted in aqueous solutions with varying NaCl concentrations, 150-nm SiO 2 particles were selected because of their reduced gravitational settling, ensuring system stability before laser-induced assembly. SiO 2 dispersions were prepared with different NaCl concentrations at a particle concentration of 1 wt%. To study assembly dynamics and ensure clear observation, large SiO 2 particles (1 μm) were used in various solvent systems, including aqueous solutions containing surfactants and oil-based solvents. A 100-μl aliquot of each prepared solution was placed inside a square PDMS spacer (1 cm side length, about 200 μm height). The spacer was sealed with a cover glass to prevent evaporation. A fs laser beam (780 nm, 80 MHz) with a scanning area of 2 × 2 µm2 was continuously applied at the corner of the printed hollow template to guide particle assembly. After the assembly process, the substrate was sequentially washed with IPA and deionized water, followed by mild sonication to remove any residual colloidal particles surrounding the microstructure.
To create 3D structures or devices made from different particles, we used a sequential, multistep assembly procedure. Suspensions of different particles were introduced successively, with a washing step between each stage to remove excess particles and prevent cross-interference. For instance, when creating alphabet letters ‘P’ and ‘I’, a suspension of 600-nm SiO 2 was first used to assemble ‘I’. The excess suspension was then rinsed away with IPA and water to ensure clean conditions for the next step, after which a suspension of 1-µm SiO 2 particles was introduced to assemble ‘P’. By repeating these processes, several particle systems can be used to construct distinct structures at predefined locations on a single substrate or to achieve seamless integration within a single microstructure, thereby enabling the creation of multifunctional microdevices.
Finally, it should be noted that excessively strong inter-particle attraction can lead to the formation of large clusters that clog the openings, preventing complete filling of the 3D structures (Supplementary Video 15). This issue can be mitigated by optimizing the size and distribution of template openings, as shown in Supplementary Discussion 1 and Supplementary Figs. 16–18.
Template removal
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