I. Core Research Summary

• Aerosols enable the one-step fabrication of multi-scale hierarchical structures.

By tuning the ratio of methanol–water mixed solvent and the sheath‑gas pressure, a single printing process simultaneously fabricates a three‑level hierarchical structure comprising millimeter‑scale pillars, micrometer‑scale columnar bodies, and submicrometer‑scale branches, spanning five orders of magnitude in size—something that conventional templating methods cannot achieve for such a multi‑level integrated architecture.

• Branch BP electrodes achieve a dual enhancement of biocompatibility and light harvesting.

The sub‑micron branched structure on the BP microcolumn surface dramatically increases the electrochemical active area, and at the same height, the chlorophyll loading is 60% higher than that of the IO electrode. The hollow column architecture achieves a light transmittance of 26.4%, significantly exceeding the 4.4% of the IO electrode, allowing light to penetrate deep into the electrode’s algal layer and addressing the issue of light shielding inherent in porous structures.

• A leap in magnitude in optoelectronic performance

A 600-μm-high branched microcolumn electrode exhibits a photocurrent of 245 μA cm⁻² and an external quantum efficiency of 29% when the redox mediator DCBQ is added; in the absence of a mediator, the direct photocurrent is 1.91 μA cm⁻². Its performance is tenfold that of a comparable inverse opal electrode, approaching the theoretical limit.

• Rapid construction of a highly adjustable electrode library

Aerosol-based digital printing allows seamless switching among pillar heights of 50, 100, 200, 600, and 800 μm, enabling the high‑throughput fabrication of gradient electrode samples. This approach facilitates rapid establishment of a quantitative structure–photovoltaic performance relationship, demonstrating that pillar height is a key parameter for enhancing power generation.

• Stable and Long-Lasting Bio-Optoelectronic System

The microcolumnar three-dimensional structure uniformly anchors Synechocystis; after 12 hours of dark incubation, no large-scale cell detachment is observed, and the photocurrent baseline remains stable over thousands of light–dark cycles, making it well suited for long-term bioelectricity generation and CO₂ reduction applications.

II. Complete Preparation Process for Aerosol 3D Microcolumn Electrodes

(1) Preparation Process

• ITO nanopowder and a methanol/deionized water mixture were used to prepare an atomizable conductive ink; ultrasonic dispersion at 50°C for 3 hours was employed to eliminate particle agglomeration.
• Plasma cleaning of FTO conductive glass substrates to remove surface oils and contaminants.
• Optomec AJ200 aerosol system, with nitrogen as the carrier gas and a coaxial sheath gas for dual‑path focusing; by adjusting the pressure ratio to 2.4–3.5, it enables stable deposition of vertically aligned microcolumns.
• Solvent ratio controls morphology: a 60% methanol solution yields a smooth SP column, while a 70% methanol solution produces a branched BP column.
• After printing, the sample was annealed in a tube furnace at 500°C for 20 minutes to remove organic solvents and sinter it, thereby forming a continuous conductive network.
• 6. The silicon mold was filled with a culture medium containing Synechococcus, and the sample was incubated in the dark for 12 hours before performing a comprehensive suite of transmission, fluorescence, and photoelectrochemical measurements.

(II) Five Core Advantages of Aerosol-Based Processes (Comparison with Template Method and Lithography)

• One-step fabrication of multi-level, hierarchical microstructures, eliminating the need for multiple processes such as templating, etching, and release‑layer removal.

Opaline electrodes require polystyrene‑sphere templates to be infiltrated and then removed at high temperatures, a process that takes more than three days; in contrast, aerosol printing can directly fabricate a three‑dimensional hierarchical architecture in a single step, reducing the sample‑preparation time by 90%.

• The column height and surface morphology are digitally adjustable with precision.

CAD is integrated with gas flow and solvent parameters to enable the batch fabrication of 50–800 μm gradient electrodes on the same day, facilitating rapid screening of optimal photovoltaic device architectures.

• The hollow three-dimensional structure offers high light transmittance, addressing the drawback of light obstruction inherent in porous electrodes.

The large inter‑microcolumn gaps allow light to penetrate the electrode substrate, resulting in an optical absorption loss exceeding 80% for the IO‑based dense pores, compared with only 53% for the BP electrode, thereby significantly enhancing the light utilization efficiency of the underlying microorganisms.

• Contactless, gentle deposition with no mechanical damage to the substrate.

3–5 mm constant spray distance, no scratching of the FTO conductive layer, and compatibility with glass, flexible conductive films, and a variety of substrates for expanded applications.

• Low-cost, small-batch R&D; ITO spot deposition saves nanomaterials.

Only the functional electrode area is printed, and the template‑based full‑sheet浸润 process consumes large amounts of ITO slurry, significantly reducing consumable costs in laboratory mechanistic studies.

III. Five Key Technological Innovations

Innovation 1: Aerosol-based innovative approach employing a multi‑stage, three‑dimensional micro‑pillar integrated printing process.

Innovation 2: A New Paradigm for Electrode Co-Optimization of Light Absorption and Biofilm Load

Innovation 3: A Rapid R&D System for Batch-Produced Graded Electrode Libraries

Innovation: Electrode Design Theory for a Breakthrough in Semi-Artificial Photosynthetic Energy Levels

Innovation 5: A Universal Manufacturing Approach for Template-Free Three-Dimensional Conductive Oxide Microstructures

IV. Current Industry Challenges in the Process and Corresponding Breakthrough Solutions

Challenge 1: The conventional template method involves lengthy processing steps and can only produce porous membranes of fixed thickness, making it impossible to control the three-dimensional height.

Challenge 2: The dense opaline pores severely block light, preventing photosynthetic microorganisms in the lower layers from absorbing sufficient illumination.

Challenge 3: A single smooth electrode has insufficient biofouling sites, resulting in a low photocurrent.

Challenge 4: The fabrication of three-dimensional oxide electrodes relies on high-temperature template etching, making it difficult to rapidly screen and optimize structural designs.

Challenge 5: The template method uses expensive consumables, and polystyrene microspheres as well as the ITO slurry are wasted to a significant extent.

V. Future Diversified Industrial Application Scenarios

1 Semi-Artificial Photosynthetic Bioelectricity-Generating Chip

2. Three-dimensional electrode for photocatalytic CO₂ reduction

3. Bio-Optoelectronic Sensing Array for Aquatic Environments

4 Micro-scale photoelectrochemical water-splitting devices

5. University-level Biophotocatalysis R&D Platform

Figure 1. Next-generation electrodes for bio‑photoelectrochemistry. a, Schematic of a bio‑photoelectrochemical cell that uses a photosynthetic biomembrane as the photocatalyst for bio‑solar power generation. Electron flow derived from photosynthetic water oxidation is collected at the anode and transferred to the cathode, where oxygen is reduced to water. Inset: Color SEM image of a microcolumn electrode printed with ITO nanoparticles and loaded with cyanobacterial cells; scale bar = 10 µm. b, Schematic of the cyanobacteria–anode interface. Light is captured by Photosystem II (PSII), which oxidizes water, releasing electrons, O₂, and H⁺. These electrons are passed through the photosynthetic electron transport chain to Photosystem I (PSI), which converts absorbed light energy into electron flow. The electrons can then be channeled into various intracellular reduction pathways, including carbon dioxide fixation in the Calvin–Benson cycle. A portion of the electrons generated during photosynthesis undergoes extracellular electron transfer, a process that can be enhanced by exogenous electron carriers such as DCBQ, enabling the delivery of electrons to the electrode surface⁶. c, Photocurrent outputs produced by specific key cyanobacterial electrodes corresponding to different generations of electrode designs⁷, 18,35–38. d, Library of aerosol‑jet‑printed microcolumn ITO electrodes demonstrated in this study. e, SEM image of the printed microcolumn array electrode (top view). f, SEM images of the printed microcolumn array electrode at different heights (side view); diameter = 20 µm, spacing = 100 µm.

Figure 2. Aerosol jet printing process for microcolumn array electrodes. a) Schematic of the printing procedure: ITO nanoparticles dispersed in a methanol–water ink precursor are atomized by an ultrasonic nebulizer, then entrained in a nitrogen carrier gas stream and subjected to aerodynamic focusing via a coaxial nitrogen sheath flow; finally, the microcolumnar ITO structures are deposited onto an FTO‑coated glass substrate. b) Printing parameters for microcolumn fabrication: the gas‑focusing pressure ratio was identified as a key control parameter for shaping the column morphology. Inset: SEM image of a representative printed column (including its base). c) Printing parameters for submicron‑scale roughness: the optimal operating window between the gas‑focusing pressure ratio and the methanol content of the ink was determined, enabling the formation of branched microdroplet architectures. d) SEM image of a typical smooth microcolumn ITO (SP‑ITO) electrode (height 600 µm, diameter 20 µm); at higher magnification, nanoscale features arising from nanoparticle packing are visible; this electrode was fabricated using a focusing pressure ratio of 2.5 and a methanol volume fraction of 60%. e) SEM image of a typical branched microcolumn ITO (BP‑ITO) electrode (height 600 µm, diameter 20 µm); at high magnification, microbranching structures containing submicron‑sized droplets are evident, with further magnification revealing nanoscale features of the constituent nanoparticles. BP‑ITO was prepared under conditions of a gas‑focusing pressure ratio of 2.5 and a methanol volume fraction of 70%. Error bars represent the standard error of the mean (n = 3).

Figure 3. Printed microcolumn electrodes exhibit high optical transmittance and cell‑loading capacity. a, Schematic of light–electrode interactions for a bare electrode; b, Schematic of light–electrode interactions for an immobilized‑cell electrode; c, Optical transmittance and reflectance measurements of a bare electrode in air; d, Transmittance and reflectance measurements of an electrode co‑incubated with cells. All measurements were performed using vertically incident white light at 0° (1 mW·cm⁻²). The IO‑ITO electrode has a height of 50 µm, while the SP‑ITO and BP‑ITO electrodes range from 50 to 800 µm in height. A concentrated suspension of Synechococcus cells (with chlorophyll a concentration of 150 nmol·ml⁻¹ in BG11 medium) was co‑incubated with the electrodes for 12 h; prior to measurement, the electrodes were gently rinsed with fresh medium. e, EASA values determined by capacitance measurements for planar IO‑ITO, SP‑ITO, and BP‑ITO electrodes; inset: enlarged comparison of EASA values for five electrodes of 50 µm height. f, Chlorophyll a loading on the electrodes after 12 h of Synechococcus incubation and gentle rinsing (normalized by EASA). g, Representative confocal microscopy image of a cross‑section of a Synechococcus‑loaded electrode (scale bar: 20 µm). h, Color scanning electron microscopy image of a Synechococcus‑loaded electrode (top view); error bars indicate the standard error of the mean (n = 3).

Figure 4. Photoelectrochemical performance of Synechococcus‑loaded electrodes. a) Representative photocurrent–voltage curves obtained from Synechococcus‑loaded BP‑ITO electrodes in the presence and absence of the exogenous electron shuttle DCBQ (1 mM). b) Summary of mediated and unmediated photoelectrochemical performance under red-light illumination (680 nm, 1 mW·cm⁻²) for Synechococcus‑loaded electrodes with different architectures, all having the same thickness (50 µm). c) Summary of unmediated photoelectrochemical performance under red-light illumination (680 nm, 1 mW·cm⁻²) for Synechococcus‑loaded SP‑ITO and BP‑ITO electrodes with varying column heights and no exogenous electron shuttle. d) Photoelectrochemical performance of the optimized Synechococcus‑loaded BP‑ITO electrode (column height 600 µm) under white-light irradiation at various intensities. Chronoamperometric measurements were conducted in BG11 medium (pH 8.5) at 25 °C, using applied potentials of 0.3 V and 0.5 V versus the standard hydrogen electrode (SHE), both in the absence and presence of DCBQ. Error bars represent the standard error of the mean (n = 3).

Figure 5. Structure–activity relationship analysis. a, Radar plots comparing the electrode structures of IO-ITO, SP-ITO, and BP-ITO (height: 50 µm); JDCBQ, photocurrent density mediated by DCBQ; MCell denotes the cell load calculated based on chlorophyll-a content. b, Spearman rank correlation matrix between the micropillar electrodes and their performance parameters. The color scale indicates the strength and direction of the correlation coefficient (R): blue denotes positive correlation, red denotes negative correlation; asterisks denote statistical significance (* P ≤ 0.1; ** P ≤ 0.05; *** P ≤ 0.01).