Automated Plasmon‐Selective Laser Writing in Mesoporous Thin Films Through Decoupling of the Initiator Absorption and Plasmon Wavelength

TL;DR

This paper introduces a method to precisely control polymerization in mesoporous films using plasmon-selective laser writing, enabling advanced applications in sensing and energy.

Contribution

The novel approach decouples plasmon wavelength from initiator absorption, enabling automated, high-precision polymer placement in mesopores.

Findings

Au nanospheres enable plasmon-induced polymerization with high reactivity and photoiniferter stability.

Wavelength separation suppresses radiative interactions, allowing plasmon-selective polymerization.

Plasmon-selective laser writing achieves lateral polymer functionalization in mesoporous membranes.

Abstract

Enhancing nanopore functionalization precision is relevant to advance future‐relevant technologies such as molecular sensing, separation, catalysis, and energy conversion. An interesting approach for high‐resolution polymerization is to use nanoscale light sources like surface plasmons. To design polymer functionalization in artificial mesopores, Au nanospheres (AuNSs) are implemented into mesoporous silica thin films, harnessing their plasmon for visible‐light near‐field‐induced polymerization initiation. AuNSs are incorporated at defined layer height, pursuing local placement of polymer along the pore depth. Using photoiniferter as photoreactive initiator for reversible‐addition‐fragmentation chain‐transfer (RAFT) polymerization requires demarcated photoreactivity of both moieties, the AuNSs and the photoiniferter. A high reactivity of AuNSs concomitant to the photostability of the photoiniferter is observed. The wavelength‐dependence of the polymerization initiation is investigated, focusing on wavelength separation of iniferter absorption and plasmon generation. Wavelength‐dependent cooperativity effects between AuNSs, the photoiniferter, and the photocatalyst are explored to understand potential radiative and nonradiative polymerization induction mechanisms of the plasmon‐induced polymerization. Ultimately, plasmon‐selective polymerization is integrated into direct laser writing, affording precise lateral polymer functionalization of mesoporous membranes. Aiming at further miniaturized polymer functionalization, a strategy to attain spatial control over polymer placement in an automated process, paving the way to directed ionic movement through mesoporous membranes, is provided.

Plasmon‐selective direct laser writing is demonstrated in mesoporous silica thin films containing plasmonic Au nanospheres. The key to attain plasmon‐selective polymerization is decoupling the plasmon wavelength from the absorption profile of coreactants, hence suppressing mutual radiative interactions. Iniferters, photostable at the incident wavelength, enhance the polymerization while coreactants photoreactive under identical conditions override the plasmon‐selectivity.© 2026 WILEY‐VCH GmbH

Figures

a) Transparent and iniferter‐grafted silica‐Au‐P3TCMPA thin films serving as mesoporous matrices for plasmon‐selective photopolymerization, and chemical structure of components/reactants. b) Induction mechanisms of plasmon‐selective polymerization, i.e., hot electron generation (e−), plasmonic heating (ΔT), and elastic reradiation (hν, improbable), and strategy enabling plasmon‐selective polymerization, based on the decoupling of iniferter absorption and the plasmon wavelength. c) Transition electron micrograph of a silica‐Au composite thin film, with the AuNSs being observable as dark spheres. d) Normalized UV/Vis spectra of all photoreactive species, i.e., P3TCMPA (2.5 mmol L−1 in DMSO, absorbance (446 nm) = 0.099), silica‐Au composite thin film in DMSO (absorbance (545 nm) = 0.201), silica‐Au‐P3TCMPA thin film in DMSO (absorbance (552 nm) = 0.222), and ZnTPP (10 μmol L−1* in DMSO, absorbance (560 nm) = 0.119) in relation to the emissive profile of the 561 nm laser; *) for ZnTPP‐catalyzed photopolymerizations a ZnTPP concentration of 14.7 mmol L−1 was used.

Fluorescence microscopy of a–d) silica‐Au‐P3TCMPA and e–h) silica‐ref‐P3TCMPA after ZnTPP‐catalyzed laser writing in METAC/DMSO (561 nm, 1.5 s per spot, variable laser output) and after colorization with Alexa 488, leading to visible arrays (inner and outer double grids) of PMETAC polymer spots. The denoted relative laser powers correspond to the following laser output powers: 5% = 1.8 mW, 10% = 3.6 mW, 20% = 7.1 mw, 50% = 17.3 mW, 100% = 34.7 mW. i) Averaged gray values (GVref,) of the respective grids, referenced to the background GV and assigned to the laser output power applied. k) ATR‐IR spectra of silica‐Au‐P3TCMPA and silica‐ref‐P3TCMPA films, exposed to 590 nm LED irradiation (1 h) while immersed in METAC/DMSO/ZnTPP solution.

Fluorescence microscopy of a) silica‐Au‐P3TCMPA, b) silica‐ref‐P3TCMPA (the violet star indicates P3TCMPA functionalization), c) unfunctionalized silica‐Au composite, and d) unfunctionalized silica reference thin films after laser writing with deoxygenated METAC/DMSO (561 nm, 10 mW laser output) and colorization with Alexa 488, leading to the visible crosses 1‐4. In all samples, two cross‐like patterns of spots, within fixed distance to a reference point, were irradiated according to the conditions highlighted in e). f) ATR‐IR spectra of silica‐Au‐P3TCMPA, silica‐ref‐P3TCMPA, unfunctionalized silica‐Au composite, and unfunctionalized silica reference films, exposed to 550–570 nm LED widefield irradiation (5 h, 18 mW cm−2) while immersed in deoxygenated METAC/DMSO.

Fluorescence microscopy of silica‐Au‐P3TCMPA films after 561 nm laser writing in METAC/DMSO at 10 mW laser output power and irradiation times of a) 10 s per spot, b) 30 s per spot, c) 75 s per spot, d) 150 s per spot, upon colorization of PMETAC polymer spots with Alexa 488. e) Averaged GVref of the respective arrays of polymer spots in relation to the applied energy dose per spot. GV profiles along the transversal section of selected spots of f1) cross 5 (Figure 4d), g1) cross 1 (Figure 3a), and h1) cross 2 (Figure 3a) (white scale bars indicate 100 μm), serving as basis for determination of the individual spot widths of f2–g2) from the corresponding Gaussian fit. Due to inapplicable Gaussian modeling of the GV profile in h2), the related spot width was estimated from the GV profile, where the dashed lines indicate the limits of the spot.

Chemical structure of P3TCMPA‐a1; lowercase letters are assigned to the hydrogen atoms.

Tables

Table 1. Widths of PMETAC spots, determined for various laser writing experiments in P3TCMPA functionalized and unfunctionalized silica‐Au composite thin films and assigned to the respective fluorescence microscopy experiment, the irradiation conditions applied, the utilized objective of 60× or 100× magnification, and the presence of ZnTPP in the reaction solution.

Figure	Spot width [μm]	Irradiation time per spot [s]	Laser output power [mW]	Energy dose [J]	Remarks
S5	33.6	0.75	34.7	0.026	60×, ZnTPP catalyzed
2d	51.2	1.5	34.7	0.052	60× ZnTPP catalyzed
4a	40.7	10	10.0	0.1	100×
4df	54.3	150	10.0	1.5	100×
3a/4g	174.6	300	10.0	3.0	60×
3c	72.4	300	10.0	3.0	60×, without iniferter
3a/4h	176.1	1800	10.0	18.0	60×, estimated

Full text

Introduction

1

The highly specific surface area and spatial confinement of (functionalized) mesopores govern their multifaceted interactions with matter in the liquid medium,^[^ 1 ^]^ evolving in adsorption,^[^ 2 ^]^ chemical repulsion,^[^ 3 ^]^ size‐dependent exclusion (molecular sieving),^[^ 4, 5 ^]^ and directed transport.^[^ 6, 7 ^]^ The effects engender widespread technological relevance such as for sensing,^[^ 8, 9 ^]^ catalysis,^[^ 10, 11 ^]^ recycling,^[^ 12 ^]^ medicine,^[^ 13 ^]^ or energy conversion.^[^ 3, 14 ^]^ All these technologies strongly rely on transport modulation and molecular behavior in confined space of pores. Inspired by the design and performance of biological pores,^[^ 15, 16 ^]^ polymer functionalization of porous materials is strongly investigated.^[^ 6, 17, 18 ^]^ While transport gating has been widely implemented using stimuli‐responsive polymers,^[^ 19, 20 ^]^ recent advancements in the polymer functionalization of mesoporous matrices involve the grafting of multifunctional block*‐co*‐oligomers with controlled chain sequences into mesoporous silica (MPS) thin films,^[^ 21 ^]^ aiming at fine‐tuned ion permselectivity and multiresponsive pores subjected to orthogonal external stimuli. The efficiency of gating is further enhanced by the localized placement of stimuli‐responsive polymers.^[^ 22 ^]^ However, fundamental thermally and light‐induced polymer grafting approaches to MPS are locally unselective,^[^ 23 ^]^ leading to uniformly distributed polymer throughout the mesoporous matrix. Techniques to achieve locally selective polymer grafting in‐depth of mesopores are known only to a limited extent. Nanoscale precision in polymer placement along the layer thickness was demonstrated through layer‐wise polymer functionalization of MPS multilayers,^[^ 24 ^]^ enabled through differing surface structures of the respective layers.^[^ 25 ^]^ The feasibility of this approach relies on successful layer stacking and the distinct chemical reactivity of each layer, while its local precision depends on practicable layer thicknesses ≥10 nm and is limited to this dimension.^[^ 26 ^]^ Other approaches focus on the automated photolithography technique of direct laser writing (DLW).^[^ 27, 28 ^]^ Maskless DLW enables additive manufacturing of nanoscale functionalities at the solid interfaces of dense^[^ 29 ^]^ and mesoporous materials^[^ 30 ^]^ through the exposure of photoreactive precursors to a focused laser beam.^[^ 27 ^]^ In photosensitive matrices, such as curable photoresin formulations, advanced DLW allows for the formation of 3D polymer nanostructures, defined through their lateral and axial extension as well as their separation to neighboring nanostructures, i.e., resolution.^[^ 31 ^]^ The volumetric polymer units, so‐called voxels, are formed in the focal point of a pulsed laser beam where the local intensity exceeds the threshold for two‐photon absorption (2PA) of the photoreactive species.^[^ 31 ^]^ The technique renders nanostructures with spatial extensions well below Abbe's diffraction limit accessible, enabling lateral structure widths down to 7 nm at 33 nm resolution.^[^ 32 ^]^ However, according to the linear‐exposure model, the elliptically shaped polymer voxels resulting from multiphoton absorption nanolithography exhibit axial dimensions up to seven times greater than their lateral dimensions,^[^ 33 ^]^ typically limiting the axial resolution of the procedure to ≈100 nm in more recent approaches.^[^ 34 ^]^ Strategies to reduce axial polymer voxel lengths in DLW are inspired by the stimulated emission depletion (STED),^[^ 35 ^]^ enabling minimum axial lengths of polymer voxels of 40 nm in photoresin matrices.^[^ 36 ^]^ In contrast to high‐resolution microscopy, including STED, relying on complex irradiation setups, plasmonic nanoparticles implemented in membranes provide an inherent nanoscale energy source in a specific location applicable for localized photoreactions.^[^ 37, 38, 39 ^]^ Plasmonic structural elements allow for local polymerization initiation even within porous materials and thus for hybrid material fabrication as demonstrated in our previous work.^[^ 40, 41 ^]^ For instance, laser irradiation of Au surfaces in the Kretschmann configuration permits plasmon‐induced dye‐sensitized polymerization in proximity to the metal surface.^[^ 41 ^]^ Moreover, Au nanoparticles enable nanolocal polymerization initiation inside their localized surface plasmon (LSP) near‐field with a local resolution of <10 nm from the particle surface, leading to the locally confined formation of polymer shells from the particle surface with accessible thicknesses ≤3 nm.^[^ 37 ^]^ Plasmon‐initiation was combined with controlled radical polymerization (CRP) protocols, such as atom transfer radical polymerization (ATRP)^[^ 37 ^]^ and reversible‐addition‐fragmentation chain‐transfer (RAFT) polymerization,^[^ 42 ^]^ to obtain narrow distributions of polymeric chain lengths. As these polymerizations are in principle initiated by the laser far‐field as well as by the plasmon near‐field irradiation, plasmon‐selective initiation is typically achieved below the reported upper energy threshold.^[^ 40 ^]^ Only below these energy thresholds, polymerization is limited to the enhanced plasmonic near‐field and hence near‐field selective, while the competing far‐field polymerization remains nonmeasurable.^[^ 40 ^]^ The resulting narrow energy range below the threshold energy of previous plasmon‐selective polymerization protocols limits their applicability.

Importantly, the mechanism of plasmon‐induced polymerization depends on reaction parameters as well as the identity of coreactants and is subject of current research,^[^ 43, 44 ^]^ focusing on the 1) radiative pathway, 2) the electronic pathway, and the 3) thermoplasmonic pathway as potential induction mechanisms.^[^ 43 ^]^ In many cases, the underlying induction mechanisms remain unclear, with ongoing controversy surrounding the specific contributions of the electronic and thermoplasmonic effect to plasmon‐induced reactions.^[^ 43, 45 ^]^ As photoreactive species, plasmonic nanoparticles become reactive upon photoexcitation for the lifetime of their excited state (5–100 fs), which decays concomitant to the dephasing of the plasmon's electronic oscillation.^[^ 46 ^]^ Radiative plasmon decay is dominated by elastic reradiation through the plasmon and results in the formation of a strongly enhanced optical near‐field in proximity of the plasmon.^[^ 46 ^]^ Nonradiative plasmon decay proceeds through (hot) electron generation and simultaneous heat dissipation (thermoplasmonic effect).^[^ 47 ^]^ Resulting from Landau damping or chemical interface damping, hot electrons may be ejected from the plasmon and transferred to adjacent molecule orbitals, engaging in chemical reactions or catalytic processes.^[^ 46 ^]^ Simultaneously, the heat generated through the rapid relaxation of the plasmon's electronic oscillation is dissipated to the nanoparticle's lattice and environment.^[^ 45 ^]^ Experimental studies show dominant radiative interactions between the plasmonic species and the reactants during dye‐sensitized polymerizations in presence of the photocatalysts Ir(piq)2(tmd)^[^ 37 ^]^ and Eosin Y,^[^ 38, 43 ^]^ favored through overlapping absorption profiles of the plasmonic species and the photocatalysts. The examples show that plasmonic nanoparticles and photocatalysts with overlapping absorbance are commonly selected to attain plasmon‐selective polymerizations. In contrast, electronic interactions between the plasmonic species and the reactants are favored in the presence of compatible molecular orbitals of similar energy^[^ 44 ^]^ and can be enforced through the omission of photocatalyst mediators.^[^ 43 ^]^ Thermoplasmonic reactions can be promoted by increasing the incident irradiation power, which affects the nanoparticle temperature in a nonlinear fashion.^[^ 48 ^]^ In this context, a nonproportional temperature increase by 27 K (2 mW of laser irradiation) against 120 K (8 mW laser irradiation) relative to the environment was predicted for plasmonic Au nanospheres (AuNSs) with a diameter of 80 nm, possibly enabling thermal polymerization initiation for methacrylates.^[^ 48 ^]^ Due to the concomitant occurrence of electronic and thermoplasmonic effects, the possibility for synergetic effects, i.e., the enhanced generation of hot electrons at increased temperature, needs to be considered.^[^ 48 ^]^ Especially, in complex reactive systems, including additional photoreactive species besides the plasmonic nanoparticles, the outlined plasmonic reaction mechanisms may coexist and lack methodical predictability. Interestingly, this synergy has not been investigated for photopolymerizations, although it bears the potential for selective plasmon‐induced reactions also at high irradiation energy and thus without threshold, enabling plasmon‐selective, automated, nanoscale laser writing in mesopores.

Here, we demonstrate a new platform combining DLW with LSP initiation to functionalize mesoporous ceramic layers in an automated manner. We systematically investigate the wavelength‐dependence of plasmon‐selective laser writing and demonstrate plasmon‐selective initiation upon separating the photoreactivity of plasmonic AuNSs and the photoiniferter, thereby suppressing adverse far‐field polymerization. In this way, we show the crucial role of harnessing the different energy transfer mechanisms of plasmons for plasmon‐selective and automated polymer writing independently of energy thresholds and in the absence of far‐field polymerization. The impact of photoiniferter and photocatalyst on the overall reactivity and potential plasmonic induction mechanisms is systematically assessed, and optimal reaction conditions for plasmon‐selective near‐field‐induced laser writing using this mechanistic concept are deduced. We expect this new concept for plasmon‐selective, automated laser writing in mesopores, these mechanistic insights, and the developed design principles for automated plasmon‐induced polymer writing to impact high‐resolution polymer functionalization in general and specifically in the context of porous materials. This is expected to impact the development of functional materials, involving membranes with laser‐written patterns applicable to, e.g., oil separation,^[^ 49 ^]^ water harvesting,^[^ 50 ^]^ as well as targeted pathogen sensing,^[^ 51 ^]^ or asymmetrically laser‐modified membranes applicable to ion current rectification.^[^ 6 ^]^

Results and Discussion

2

MPS Double‐Layers as Platform for Plasmon‐Induced Free Radical and Controlled RAFT Polymerizations

2.1

To enable plasmon‐induced, automated laser writing, plasmonic AuNSs with a median particle diameter of 16 nm (Figure S1, Supporting Information) were implemented into MPS thin films to confine the initiation of the polymerization during laser writing to the particles’ plasmonic near‐field. The plasmonic near‐field modes, generated at a defined layer height of the MPS thin films, were harnessed for nanoscale local polymerization initiation in the presence of a writing laser beam, automatically scanning across a previously encoded array of x/y coordinates and controlled by the laser operating software. The nanoscale penetration depth of the plasmon locally restricts the polymer formation in the z direction, along the laser beam path. For a systematic mechanistic understanding, the plasmon‐selective laser writing was pursued in three sample configurations based on MPS‐Au composite thin films (Figure 1a–c), i.e., 1) pristine silica‐Au composite films, 2) silica‐Au composite films with surface‐bound photoiniferter as coinitiator, and 3) silica‐Au composite films with surface‐bound photoiniferter as well as dissolved photocatalyst absorbing at the laser wavelength. The addition of photoiniferter and photocatalyst to silica‐Au composite films aimed at enhanced polymer formation, leveraging potential interactions between the plasmonic near‐field enhancement of the AuNSs and the initiator and photocatalyst. Corresponding to the absorption maxima of pristine as well as of iniferter‐functionalized silica‐Au composite films at 545 nm and 552 nm, an overlapping incident wavelength of 561 nm was selected to induce near‐maximum activity to the AuNSs during laser writing (Figure 1d). A 590 nm light‐emitting diode (LED) and a metal halide lamp (Figure S2, Supporting Information), covering the same wavelength regime as the 561 nm laser, were utilized for widefield irradiation of MPS thin films, enabling large area irradiation of the samples for attenuated total internal reflection‐infrared (ATR‐IR) spectroscopy characterization. To prevent undesired direct iniferter‐initiated polymerization, a photoiniferter being photostable at the incident wavelength of 561 nm was required. As current research shows a bathochromically shifted photoreactivity relative to the absorption spectra of various classes of photoreactive species,^[^ 52, 53 ^]^ the prerequisites for a suitable photoiniferter have evolved from the criterion of negligible absorbance at the incident wavelength to an actual photoreactivity close to zero at 561 nm. Meeting both prerequisites, the trithiocarbonate‐based photoiniferter (2‐(propylthiocarbonothioylthio)‐2‐methylpropanoic acid (P3TCMPA), with an absorption maximum shifted by 115 nm from the incident wavelength of 561 nm to 446 nm (Figure 1d) while exhibiting no polymer formation in free solution experiments at 550–570 nm irradiation (Figure S3a, Supporting Information), was selected as photostable initiator. Possessing a short alkyl chain in contrast to its prominent structural derivative (2‐(dodecylthiocarbonothioylthio)‐ 2‐methylpropanoic acid (D3TCMPA),^[^ 30, 54, 55 ^]^ P3TCMPA requires a reduced space inside the confinement of the silica mesopores. The photocatalyst 5,10,15,20‐tetraphenyl‐21H,23H‐porphine zinc(II) (ZnTPP), associated with mild reaction conditions and oxygen‐tolerant photoinduced electron transfer (PET)‐RAFT polymerization,^[^ 56 ^]^ was furthermore selected due to its overlapping photoactivity with AuNSs at the incident wavelength of 561 nm (Figure 1d), facilitating radiative interactions between photoexcited AuNSs and ZnTPP to enhance the polymerization. The absorbance of silica‐Au‐P3TCMPA thin films in the ZnTPP‐containing polymerization solution was kept below 1.4, but 13.8‐fold higher than the absorbance of the AuNSs‐containing silica‐Au‐film in ZnTPP‐free solution (Figure S3b, Supporting Information). According to the literature,^[^ 46 ^]^ the overlapping absorption profiles between AuNSs and ZnTPP are expected to provide a direct excitation pathway for ZnTPP via elastic radiative re‐emission originating from photoexcited AuNSs. To distinguish plasmon‐selective near‐field polymerization from plasmon‐independent far‐field polymerization, all photopolymerization experiments were performed in MPS‐Au composite and Au‐free silica reference films, of which only the former are applicable for plasmon‐selective polymerization. Hence, exclusive reactivity in silica‐Au composite films is considered as an indicator for plasmon‐selective polymerization. In addition, plasmon‐selective polymerization is characterized by not coinciding with far‐field‐induced iniferter photoinitiation or light‐induced auto‐polymerization.

a) Transparent and iniferter‐grafted silica‐Au‐P3TCMPA thin films serving as mesoporous matrices for plasmon‐selective photopolymerization, and chemical structure of components/reactants. b) Induction mechanisms of plasmon‐selective polymerization, i.e., hot electron generation (e−), plasmonic heating (ΔT), and elastic reradiation (hν, improbable), and strategy enabling plasmon‐selective polymerization, based on the decoupling of iniferter absorption and the plasmon wavelength. c) Transition electron micrograph of a silica‐Au composite thin film, with the AuNSs being observable as dark spheres. d) Normalized UV/Vis spectra of all photoreactive species, i.e., P3TCMPA (2.5 mmol L−1 in DMSO, absorbance (446 nm) = 0.099), silica‐Au composite thin film in DMSO (absorbance (545 nm) = 0.201), silica‐Au‐P3TCMPA thin film in DMSO (absorbance (552 nm) = 0.222), and ZnTPP (10 μmol L−1 in DMSO, absorbance (560 nm) = 0.119) in relation to the emissive profile of the 561 nm laser; ) for ZnTPP‐catalyzed photopolymerizations a ZnTPP concentration of 14.7 mmol L−1 was used.

MPS thin films were fabricated through dip‐coating and sol–gel chemistry in the evaporation‐induced self‐assembly (EISA) process.^[^ 57 ^]^ In our previous work, the as‐fabricated MPS thin films with and without plasmonic nanoparticles were demonstrated to maintain their structural integrity during surface‐initiated RAFT polymerizations, as evidenced by transition electron microscopy (TEM)^[^ 40 ^]^ and ellipsometry.^[^ 30 ^]^ Double‐layered, Au‐free silica reference films resulted from consecutive dip‐coating and calcination. For the preparation of silica‐Au composite films, the outer surface of an MPS single‐layer with templating agent was functionalized with the amine linker 3‐aminopropyldimethylmethoxysilane for the subsequent immobilization of AuNSs through electrostatic interactions. The resulting AuNSs‐conjugated silica films were subjected to dip‐coating of a second MPS layer and subsequent calcination, providing the double‐layered sandwich structure of silica‐Au composite films with the AuNSs being physically entrapped in between both layers (Figure 1a). The exchange of formerly employed Ag/Au alloy nanospheres^[^ 40 ^]^ by AuNSs significantly increased the chemical and thermal stability of silica‐Au composite films to temperatures of 350 °C, as verified by the retained spherical shape of the AuNSs in TEM (Figure 1c) and the sustained absorption maximum of silica‐immobilized AuNSs at 545 nm in UV/Vis spectroscopy (Figure 1d). To obtain iniferter‐grafted MPS‐Au‐P3TCMPA and silica‐ref‐P3TCMPA thin films, silica‐Au composite and reference films were functionalized with P3TCMPA following the reported grafting‐from approach (Figure 1a).^[^ 40 ^]^ Subsequent CO_2_ plasma treatment was applied to remove iniferter species from the outer planar surface of the MPS thin films, thus limiting the polymerization to inside the mesopores.^[^ 58, 59 ^]^ The MPS‐Au composite and Au‐free silica reference thin films are characterized by an averaged thickness of 291 nm and 270 nm while exhibiting an average pore diameter of 6.0 ± 1.3 nm and 4.4 ± 0.9 (Figure S4, Supporting Information), respectively. The thickness of the bottom MPS layer in silica‐Au composite films, determined to be 181 nm, defines the fixed z position of AuNSs in the mesoporous double‐layer cross‐section.

Reactivity of Silica‐Au Composite and Reference Films in ZnTPP‐Catalyzed Laser Writing

2.2

To understand the influence of process parameters and mesoporous film structure on plasmon‐induced laser writing, spot arrays (Figure 2a–h) in the presence and absence of the plasmonic AuNSs were written into MPS thin films. The spot arrays consisted of a so‐called outer grid with array‐specific reference spots, irradiated at maximum laser power, and an inner grid of 3 × 3 spots (Figure 2), irradiated at variable laser power. The polymerization was performed using [2‐(methacryloyloxy)ethyl]trimethylammonium chloride (METAC) as monomer. To visualize the local formation of non‐fluorescent poly([2‐(methacryloyloxy)ethyl]trimethylammonium chloride) (PMETAC) in MPS thin films induced by laser irradiation, the films were extracted to remove unbound monomer and subsequently incubated in an aqueous Alexa 488 fluorophore solution to allow adsorption of the fluorophore to the oppositely charged polyelectrolyte as demonstrated in literature.^[^ 30 ^]^ Upon subsequent aqueous extraction of the MPS thin films, the fluorophore remained adsorbed at the polyelectrolyte sites, whereas it was largely washed out from sites without polyelectrolyte. The site‐selective retention of the fluorophore after extraction was demonstrated in fluorescence microscopy, showing a significant contrast between the irradiated and nonirradiated sites (Figure 2a–h). Assuming a proportional relationship between the quantity of adsorbed molecules and polymeric amount (Figure 4a–e), fluorescence intensity‐related gray values (GVs) may provide an indirect measure for polymer amounts within a defined region of interest as demonstrated in literature.^[^ 60 ^]^ To correct for background fluorescence as originated by residual fluorophore, GVs were generally referenced to background GVs, as reflected by GV_ref_ (experimental section). Apart from enhancing plasmon‐induced polymerization, the utilization of ZnTPP as a known oxygen‐scavenging species in dimethyl sulfoxide (DMSO)^[^ 61 ^]^ aims to render the laser writing process oxygen‐tolerant. Due to the overlapping photoactivity of AuNSs and ZnTPP at the incident wavelength of 561 nm (Figure 1d), simultaneous plasmon‐induced polymerization and far‐field induced ZnTPP‐initiated PET‐RAFT polymerization coincide. Therefore, laser writing experiments in presence of ZnTPP focused on investigating the extent of polymerization enhancement achievable through the interaction between AuNSs and ZnTPP.

Fluorescence microscopy of a–d) silica‐Au‐P3TCMPA and e–h) silica‐ref‐P3TCMPA after ZnTPP‐catalyzed laser writing in METAC/DMSO (561 nm, 1.5 s per spot, variable laser output) and after colorization with Alexa 488, leading to visible arrays (inner and outer double grids) of PMETAC polymer spots. The denoted relative laser powers correspond to the following laser output powers: 5% = 1.8 mW, 10% = 3.6 mW, 20% = 7.1 mw, 50% = 17.3 mW, 100% = 34.7 mW. i) Averaged gray values (GVref,) of the respective grids, referenced to the background GV and assigned to the laser output power applied. k) ATR‐IR spectra of silica‐Au‐P3TCMPA and silica‐ref‐P3TCMPA films, exposed to 590 nm LED irradiation (1 h) while immersed in METAC/DMSO/ZnTPP solution.

Successful laser writing in silica‐Au‐P3TCMPA and silica‐ref‐P3TCMPA films was achieved for 561 nm laser irradiation with an output power of 1.8–34.7 mW at two different irradiation times of 1.5 s (Figure 2a–h) and 0.75 s (Figure S5, Supporting Information) per spot. A twofold increased irradiation time from 0.75 s to 1.5 s per spot results in an increase of GV_ref_ by a factor of 3.0 for silica‐Au‐P3TCMPA films and by a factor of 2.4 for the Au‐free silica‐ref‐P3TCMPA films at the maximum laser power of 34.7 mW (Figure 2i, S5, Supporting Information), showing a significant time‐dependency of the polymerization process. In general, the GV_ref_ values increase only slightly with increasing laser power, approaching an apparently constant polymer amount and dye adsorption at laser powers between 17.3 and 34.7 mW, which is observed at both irradiation times, i.e., 0.75 s (Figure S5, Supporting Information) and 1.5 s (Figure 2i). The trend appears less significant for silica‐ref‐P3TCMPA (Figure 2i) but might be masked by the increased standard deviation. The slower increase of GV_ref_ with increasing laser power for silica‐Au‐P3TCMPA and silica‐ref‐P3TCMPA films may indicate photocatalyst self‐quenching which in case of ZnTPP would proceed through triplet–triplet annihilation.^[^ 62 ^]^ The GV_ref_ ratios of silica‐ref‐P3TCMPA:silica‐Au‐P3TCMPA tend to converge at lower irradiation powers, reaching a ratio of 0.47 at 1.8 mW and a ratio of 1.06 at 3.6 mW laser power for an irradiation time of 1.5 s per spot, while clearly indicating a significantly higher written PMETAC amount for the silica‐ref‐P3TCMPA films at laser powers >3.6 mW, reaching a ratio of 2.03 at 34.7 mW for an irradiation time of 1.5 s per spot (Figure 2i). Thus, at low laser power, a similar reactivity of silica‐Au‐P3TCMPA and silica‐ref‐P3TCMPA films is observed in contrast to a superior reactivity of Au‐free silica‐ref‐P3TCMPA films at higher laser powers. Consequently, no enhancement of the plasmonic field was observed. This laser‐power dependent polymer formation at 1.5 s is confirmed using an irradiation time of 0.75 s. The GV_ref_ ratios silica‐ref‐P3TCMPA:silica‐Au‐P3TCMPA of 1.37 at a laser power of 1.8 mW, 1.18 at 3.6 mW, 0.97 at 7.1 mW, 3.22 at 17.1 mW, and 2.52 at 34.7 mW were observed, as well indicating the higher reactivity of silica‐ref‐P3TCMPA as compared to silica‐Au‐composite films at elevated laser powers here of >7.1 mW.

The decreased GV_ref_ for silica‐Au‐P3TCMPA in comparison to silica‐ref‐P3TCMPA films contradicts assumptions of universal plasmon‐enhanced polymerization through field enhancement of AuNSs and overlapping absorption with ZnTPP. To validate the polymer amount indicated by the GV_ref_ values, the polymerization in silica‐Au‐P3TCMPA and silica‐ref‐P3TCMPA thin films was simultaneously performed under 590 nm widefield irradiation at two power levels, addressing a wavelength regime similar to the 561 nm laser, and analyzed through ATR‐IR spectroscopy. In accordance with 561 nm laser writing and GV analysis, higher PMETAC amounts for Au‐free silica‐ref‐P3TCMPA films in comparison to silica‐Au‐P3TCMPA films were detected after irradiation for 60 min, irrespective of the irradiation power as indicated through the PMETAC‐specific C=O vibrational band at ≈1730 cm^−1^ ^[^ 40 ^]^ (Figure 2k). This higher polymer amount for Au‐free silica‐ref‐P3TCMPA films as compared to AuNS‐containing films may result from inherent structural differences upon AuNS integration, affecting, e.g., pore order and connectivity, reactant accessibility, and diffusion in silica‐Au composite films of reduced porosity. According to ellipsometry/grazing‐incidence small‐angle X‐ray scattering (GISAXS) measurements, the porosity of silica‐Au composite films is determined to be 44%/49% as compared to the porosity of 51%/55% of Au‐free silica reference films. GISAXS reveals the increased inhomogeneity of silica‐Au composite films in contrast to Au‐free silica reference films, indicating a slightly increased lateral disorder in pore size and inter‐pore distances which especially appears potentiated in‐depth of the film (Figure S6, Supporting Information). The increased pore disorder of silica‐Au composite films is considered to deteriorate the overall pore interconnectivity and permeability, potentially affecting reactant supply during iniferter grafting and photopolymerization. Comparing Ag/AuNSs‐containing MPS bilayers to the metal particle‐free reference, 1.8‐fold higher diffusion coefficients in the metal‐free reference than in the Ag/AuNS‐containing composite films were derived from [Ru(NH_3_)6]^2+/3+^‐based cyclic voltammetry as part of our previous work.^[^ 40 ^]^ Hence, the incorporation of metal nanospheres into MPS double‐layers is related to an increased pore disorder and lower porosity according to GISAXS (Figure S6, Supporting Information). Additionally, ATR‐IR spectroscopy shows that the structural differences between silica‐ref‐P3TCMPA and silica‐Au‐P3TCMPA films result in a lower monomer uptake of the AuNS‐composite film (Figure S7, Supporting Information). Interestingly, the presumably decelerated reactant diffusion in silica‐Au‐P3TCMPA as compared to Au‐free silica‐ref‐P3TCMPA films only measurably affects the polymerization performance at elevated laser powers ≥7.1 mW (1.5 s per spot) and ≥17.3 mW (0.75 s per spot), suggesting that monomer supply constraints are limited to increased laser powers and thus increased monomer conversion. This nicely demonstrates how polymerization in confined space can be driven into transport‐limited regimes by tuning irradiation power and catalyst activity for example. Contrarily, sufficient monomer supply concomitant to a facilitated AuNSs accessibility, both favored by reduced laser powers as well as reduced irradiation times, may promote the contribution of plasmon‐induced polymerization to ZnTPP‐catalyzed polymerization, resulting in a mixed near‐field and far‐field polymerization. Hence, an increasingly locally unselective polymer distribution can be anticipated under higher irradiation power in the presence of catalysts absorbing at the far‐field irradiation wavelength, which clearly limits plasmon selectivity for polymerization induction.

Selective Plasmon‐Induced Laser Writing While Preventing Direct Iniferter Photon Absorption

2.3

Targeting exclusive plasmon‐induced polymerization, laser writing experiments at 561 nm were performed in the presence of 1) AuNSs and P3TCMPA (Figure 3a), exploring potential cooperativity effects between field enhancement of plasmonic AuNSs and the photoiniferter, in the presence of 2) AuNSs without initiator (Figure 3b), while 3) Au‐free P3TCMPA functionalized (Figure 3c) and Figure 4). Unfunctionalized MPS thin films (Figure 3d) served as references. For exclusive plasmon‐induced polymerization, without an influence of the far field irradiation, in the presence of initiator, the decoupling of plasmon wavelength and iniferter excitation seemed to be essential (Figure 1b). Facilitating the retrieval of the polymer spots using fluorescence microscopy, the films were irradiated in cross‐like patterns within fixed distance from a reference point (Figure 3e). It has to be considered that by removing ZnTPP, the ZnTPP/DMSO‐mediated oxygen tolerance of laser writing is suspended. Minimal oxygen exposure was ensured through deoxygenation of the reaction solution and performing laser writing in a sealed compartment.

Fluorescence microscopy of a) silica‐Au‐P3TCMPA, b) silica‐ref‐P3TCMPA (the violet star indicates P3TCMPA functionalization), c) unfunctionalized silica‐Au composite, and d) unfunctionalized silica reference thin films after laser writing with deoxygenated METAC/DMSO (561 nm, 10 mW laser output) and colorization with Alexa 488, leading to the visible crosses 1‐4. In all samples, two cross‐like patterns of spots, within fixed distance to a reference point, were irradiated according to the conditions highlighted in e). f) ATR‐IR spectra of silica‐Au‐P3TCMPA, silica‐ref‐P3TCMPA, unfunctionalized silica‐Au composite, and unfunctionalized silica reference films, exposed to 550–570 nm LED widefield irradiation (5 h, 18 mW cm−2) while immersed in deoxygenated METAC/DMSO.

Upon irradiation at relatively high energy ≥3 J per spot, the intended cross‐like PMETAC spot patterns were exclusively observed for the plasmonic silica‐Au composite and the initiator containing silica‐Au‐P3TCMPA thin films (Figure 3a,b), demonstrating the essential role of the AuNSs in initiating the polymerization. The initiator containing silica‐ref‐P3TCMPA and pristine silica reference thin films, both without plasmonic AuNSs, remained inactive, not showing any polymer formation under 561 nm irradiation (Figure 3c,d). This clearly demonstrates plasmon‐selective polymer formation when using 561 nm irradiation at which a LSP is generated while the photoiniferter P3TCMPA does absorb light. Strongly enhanced GV_ref_ values by a factor of 37.2 (cross 1:cross 3, Figure 3a,b) were measured for silica‐Au‐P3TCMPA films as compared to iniferter‐free silica‐Au composite films, indicating cooperative effects between the AuNSs plasmon near field and P3TCMPA initiation. Nevertheless, polymerization also occurs without the presence of the initiator, indicating a polymerization mechanism beyond AuNSs‐P3TCMPA interactions. But the locally unselective far‐field polymerization, as resulting from direct photoexcitation of P3TCMPA, is successfully suppressed in all cases. The blurred shape of the colorized PMETAC crosses 1 and 2 in Figure 3a is associated with the prolonged irradiation times of up to 30 min per spot, which promote the diffusion of active species, i.e., iniferter radical fragments, through the mesopores, resulting in a reduced resolution of the polymer pattern. In accordance with fluorescence microscopy, the PMETAC formation in silica‐Au‐P3TCMPA thin films was verified through the presence of the methacrylate‐specific C=O vibrational band at 1730 cm^−1^ ^[^ 40 ^]^ in the ATR‐IR spectrum (Figure 3f). No PMETAC formation was detected in silica‐ref‐P3TCMPA films, as well as in pristine silica reference and silica‐Au composite films (Figure 3f). The absent C=O vibrational band in the IR spectrum of iniferter‐free silica‐Au composite films, despite visible polymer spots in the associated microscopy image (Figure 3b), is considered a consequence of the lower sensitivity of ATR‐IR spectroscopy in contrast to fluorescence microscopy as already discussed in a previous study.^[^ 60 ^]^

The comparison of silica‐Au composite to Au‐free silica reference films identifies AuNSs as the only viable photoreactive component of the studied MPS thin films in interaction with 561 nm irradiation. The formation of PMETAC in silica‐Au composite films without the presence of iniferter (Figure 3b) substantiates the occurrence of nonradiative interactions between the plasmon and the double bond of METAC, indicating that the presence of a photoinitiator, here the iniferter P3TCMPA, is no fundamental prerequisite for plasmon‐induced polymerization. Given a significant wavelength gap of 336 nm and 99 nm between the absorption maxima of AuNSs/METAC and AuNSs/P3TCMPA, respectively, radiative interactions between both species, as reported for the pair AuNSs/Eosin Y with a wavelength gap of 2 nm,^[^ 43 ^]^ appear highly improbable (Figure S8, Supporting Information). The demarcated photoreactivity of AuNSs and P3TCMPA in silica‐Au‐P3TCMPA films enables plasmon‐selective polymerization independent of irradiation energies up to at least 64.8 kJ cm^−2^. In contrast, the overlapping photoreactivity of plasmonic Ag/Au alloy NSs and the iniferter 4‐cyano‐4‐[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (CDTPA), exhibiting a wavelength gap of only 7 nm (Figure S8, Supporting Information), led to a substantial energy threshold of plasmon‐selective polymerization of 0.6 J cm^−2^ in widefield irradiation experiments beyond which plasmon‐selective polymerization was superseded by undesired iniferter‐initiated far‐field polymerization.^[^ 40 ^]^ Radiative interactions between AuNSs/METAC and AuNSs/P3TCMPA through re‐emission of photons higher in energy relative to the incident photon energy, as enabled by 2PA of AuNSs, were ruled out using a continuous wave (CW) laser setup, which, in contrast to a pulsed laser setup, provides an insufficient photon density for 2PA.^[^ 63 ^]^ Hence, nonradiative interactions between AuNSs and METAC through hot electron generation or heat transfer seem probable. While hotelectron generation leads to the formation of an Au—C bond with the C=C moiety of METAC,^[^ 64 ^]^ the thermal auto‐polymerization of METAC in proximity of the AuNSs does not essentially involve covalent bonding between Au and the polymer chain. At laser irradiation powers ≥8 mW, a temperature increase of AuNSs with a diameter of 80 nm by ≈120 K was predicted, suggesting a nanolocal temperature increase in AuNSs to over 413 K at the applied laser power of 10 mW, which theoretically suffices for auto‐polymerization of the METAC‐derivate methyl methacrylate.^[^ 48 ^]^ However, a temperature increase of the polymerization solution upon irradiation could not be measured using an IR camera (Figure S9, Supporting Information), thus not providing direct evidence for thermal polymerization. Still, a nanoscopic temperature increase at the AuNS surface may remain undetected with the selected measuring method. Hence, a polymerization initiation mechanism based on electronic interactions between AuNSs and METAC, heat transfer from the AuNSs to the polymerization solution, or combinations thereof appears still conceivable.

Tuning Spot Size and Polymer Distribution by Irradiation Energy

2.4

To tune plasmon‐selective laser writing, the time‐dependent performance of plasmon‐induced polymerization, as well as the threshold conditions for measurable polymer formation during laser writing, was investigated (Figure 4a–d) while keeping the initial laser power of 10 mW constant. To quantify the lateral precision of polymer placement in plasmon‐induced laser writing and to evaluate its dependence on process parameters, the width of exemplary polymer spots was determined and related to the irradiation conditions, the participation of ZnTPP, and the magnification of the utilized objectives (Figure 4f–h, Table 1).

Fluorescence microscopy of silica‐Au‐P3TCMPA films after 561 nm laser writing in METAC/DMSO at 10 mW laser output power and irradiation times of a) 10 s per spot, b) 30 s per spot, c) 75 s per spot, d) 150 s per spot, upon colorization of PMETAC polymer spots with Alexa 488. e) Averaged GVref of the respective arrays of polymer spots in relation to the applied energy dose per spot. GV profiles along the transversal section of selected spots of f1) cross 5 (Figure 4d), g1) cross 1 (Figure 3a), and h1) cross 2 (Figure 3a) (white scale bars indicate 100 μm), serving as basis for determination of the individual spot widths of f2–g2) from the corresponding Gaussian fit. Due to inapplicable Gaussian modeling of the GV profile in h2), the related spot width was estimated from the GV profile, where the dashed lines indicate the limits of the spot.

In silica‐Au‐P3TCMPA films, the visibility of polymer spots increases with increasing irradiation time, progressing from nearly blending into the background upon 10 s of irradiation (0.1 J, Figure 4a) to becoming clearly visible upon 150 s of irradiation (1.5 J, Figure 4d). Averaged GV_ref_ values of the corresponding polymer spots (Figure 4e) increase from 0.02 (Figure 4a) to 0.24 (Figure 4d), increasing almost linearly with the irradiation energy and irradiation time. This suggests that the writing efficiency correlates with the irradiation energy and irradiation time. The widths of polymer spots are determined to be 54.3 μm (Figure 4d,f, cross 5, 1.5 J per spot), 174.6 μm (Figure 4g, cross 1, 3 J per spot), and 176.1 μm (Figure 4h, cross 2, 18 J per spot). The GV profile (Figure 4h) significantly deviates from an ideal Gaussian distribution, as expected from the laser beam intensity profile.^[^ 60 ^]^ Therefore, the related spot width was estimated from the underlying GV profile, defining the two minima beyond the bright rim as spot limits (highlighted through dashed lines in Figure 4h).

According to Figure 4a, plasmon‐selective laser writing requires a minimum energy around 7.7 kJ cm^−2^, upon which polymer formation becomes detectable under the applied conditions. The determined energy threshold for plasmon‐selective polymerization with AuNSs/P3TCMPA (99 nm wavelength gap between absorption maxima) significantly exceeds literature‐known energy thresholds for photoreactive species with overlapping photoactivity, i.e., 2 mJ cm^−2^ for the pair AuNSs/Eosin Y^[^ 43 ^]^ (2 nm wavelength gap) and 600 mJ cm^−2^ for the pair Ag/Au NSs/CDTPA^[^ 40 ^]^ (7 nm wavelength gap), for which a radiative induction mechanism is presumed. Hence, the nonradiative polymerization induction mechanism in plasmon‐selective polymerization, proposed for AuNSs/P3TCMPA, is identified as a more energy‐consuming process than radiative induction. While near‐field induced polymerization occurs in the absence of P3TCMPA (Figure 3b), the polymerization energy threshold almost reaches a tenfold increased value of ≈72.9 kJ cm^−2^ as compared to P3TCMPA‐assisted plasmon‐induced polymerization, indicating a further decrease in induction efficiency in the absence of iniferter/photoinitiator. In Figure 4e, the near‐linear growth of averaged GV_ref_ values of PMETAC spots suggests a proportional relationship between irradiation energy and grafted polymer amount, resulting from plasmon‐selective polymerizations up to an irradiation energy of 1.5 J. Up to this energy, the resulting polymer spots share a near Gaussian transversal GV profile, indicating a polymer gradient corresponding to the Gaussian intensity profile of the laser beam, as exemplary shown in Figure 4f. A further increase in irradiation energy to 3 J and to a maximum of 18 J leads to the more irregular GV polymer spot profiles (Figure 4g,h), showing a fluorophore distribution deviant from the intensity profile of the laser beam. While the GV profile of the polymer spot in Figure 4g (3 J) retains a near Gaussian shape, reaching a plateau in the spot center, the GV profile of the polymer spot in Figure 4h (18 J) exhibits two maxima at the edges with GVs receding toward the center and forming a lower local maximum in the center. This deviation of the GV profile from the laser intensity distribution (Figure 4h) may indicate an increasingly transport‐limited polymerization toward the center of the PMETAC spot with increasing irradiation energy, or laser interference effects, which could promote inhomogeneous polymer distributions along the axis, although a collimated beam was used in these experiments. Additionally, fluorescence quenching of densely packed fluorophore molecules adsorbed to the polymer matrix may have further distorted the GV profile in Figure 4h.

Table 1 compares the PMETAC spot diameters, obtained from iniferter‐assisted plasmon‐selective laser writing (Figure 4), to the spot diameters accessible through ZnTPP catalyzed and iniferter‐free laser writing, examining the influence of process parameters on polymer spot resolution. In general, the spot diameter increases with irradiation time. In the presence of ZnTPP, doubling the irradiation time from 0.75 s to 1.5 s results in a 1.5‐fold increase in spot diameter (Figure 2d, S5, Supporting Information) widths from 33.2 μm (Figure S5, Supporting Information) to 51.2 μm (Figure 2d). In the absence of ZnTPP, a 15‐fold increase in irradiation time from 10 s to 150 s is required to achieve a 1.3‐fold increase in polymer spot diameter from a minimum of 40.7 μm in Figure 4a to 54.3 μm in Figure 4d, indicating that irradiation time exerts a weaker effect on spot diameter in plasmon‐selective than in ZnTPP‐catalyzed polymerization in silica‐Au‐P3TCMPA films. One aspect explaining the stronger increase of spot widths with increasing irradiation time in presence of ZnTPP may be the facilitated diffusion of the relatively long‐lived excited photocatalyst (triplet state T_1_ exhibits a lifetime of 4.6 ms in common organic solvents)^[^ 65 ^]^ in contrast to the much shorter‐lived iniferter radical fragments (estimated lifetime <1.1 ms,^[^ 66 ^]^ i.e., the lifetime of methyl methacrylate (MMA)‐derived radicals in the polymerization with MMA) as mobile reactive species in silica‐Au‐P3TCMPA films. A second aspect is the different polymerization mechanism.

In the absence of ZnTPP, the change from the more focused 100× objective to a 60× objective leads to polymer spots expanded by a factor of 3.1 from 51.2 μm (Figure 4d) to 174.6 μm (Figure 3a) in relation to twofold increased irradiation times in silica‐Au‐P3TCMPA films, emphasizing the importance of precise optics to attain low spot diameters. At further prolonged irradiation times ≥5 min, strongly expanded and distorted polymer spots (crosses 1 and 2, Figure 4g,h) with the most pronounced spot widths are observed, indicating substantial diffusion of active species. In contrast, the more regular spot morphologies in pristine silica‐Au composite films (Figure 3b) are characterized through significantly contracted spot widths as compared to silica‐Au‐P3TCMPA films, reaching a ratio of 0.4 (cross 1:cross 3, Figure 3) despite identical irradiation conditions. The decreased lateral precision in polymer placement in silica‐Au‐P3TCMPA films as compared to iniferter‐unfunctionalized silica‐Au composite films presumably relates to the possibility for iniferter fragmentation upon plasmon interaction, providing a reactive species for a diffusion‐driven expansion of the spatial polymer distribution. Contrarily, more contracted polymer spots in pristine silica‐Au composite films indicate less favored diffusion of METAC‐derived radicals than P3TCMPA‐derived radicals as the only present mobile reactive species. The evaluation of PMETAC spot dimensions in AuNSs‐containing MPS‐Au composite thin films identifies the omission of additional radical sources, such as P3TCMPA, and dissolved photocatalyst, such as ZnTPP, the reduction of irradiation times, and the utilization of precise optics as effective measures to maximize polymer spot resolutions in plasmon‐induced, automated laser writing.

Conclusion

3

Automated, plasmon‐selective near‐field‐induced laser writing with adjustable grafted polymer amount and spot size was successfully demonstrated across two independent experimental series by combining DLW with plasmonic AuNSs, embedded in 291 nm thick MPS films. Two different plasmon‐induced polymerization mechanisms, based on photons as well as on hot electron generation/plasmonic heat transfer, were identified and used to guide polymer functionalization. With increasing laser power and in the presence of the photocatalyst ZnTPP and thus with overlapping photocatalyst absorption and plasmon wavelength, plasmon‐induced polymerization driven by radiative interactions between photoexcited AuNSs and ZnTPP was increasingly superseded by PET‐RAFT far‐field polymerization . No measurable photon‐related field enhancement effects under the applied conditions using ZnTPP and P3TCMPA were detected. Plasmon‐selective polymer grafting in the absence of far‐field polymerization was observed in the absence of photocatalyst ZnTPP and with an irradiation and plasmon wavelength outside the absorption of the P3TCMPA iniferter. Contrarily, in the absence of AuNSs and thus under far‐field laser irradiation, no polymer formation was observed. Near‐field selective polymerization also occurred in the absence of the photoiniferter and thus exclusively in the presence of the monomer METAC. A 37‐fold polymerization enhancement in the presence of P3TCMPA as compared to the absence of P3TCMPA initiator was observed. The polymerization was ascribed to nonradiative interactions between AuNSs and METAC, very probably based on hot electron generation /local heat transfer from the excited AuNSs to the monomer or the iniferter. The substantial wavelength gaps between the absorption maxima of AuNSs/P3TCMPA (99 nm) and AuNSs/METAC (336 nm) diminish the probability for radiative interactions between both species. This concept now allows plasmon‐selective polymerization, and with this, potentially nanoscale polymer grafting without the upper energy thresholdspreviously required to avoid far‐field polymerization, which is incapable of local polymer grafting. This is particularly relevant as reported threshold energies of 2 mJ cm^−2^ observed in previous studies^[^ 43 ^]^ are very low for automated laser writing procedures. Notably, the plasmon‐selective polymerization was realized at the elevated energy level of a laser beam in the kJ cm^−2^ range and integrated in a DLW protocol. The presence of ZnTPP enhances the influence of polymerization time on the written polymer spot diameters. AuNSs‐induced laser writing without additional photoreactive species allowed for a superior lateral resolution of PMETAC spots, reaching diameters reduced by 59% against P3TCMPA‐assisted laser writing, and is expected to increase the axial resolution of PMETAC grafting based on the penetration depth of plasmon near‐field modes.^[^ 38 ^]^ Through analysis of polymerization criteria, coreactant effects on yield and resolution, as well as underlying mechanisms, we now provide a pathway toward plasmon‐selective near‐field‐induced, automated DLW in spatially confined mesopores, opening new possibilities in design, transport control, and hierarchical material integration.

Experimental Section

4

4.1

4.1.1

Materials and Procedures

Propane‐1‐thiol (99%), potassium phosphate trihydrate (Reagent Plus, 99%), carbon disulfide (99%), 2‐bromo‐2‐methylpropionic acid (98%), (3‐aminopropyl)triethoxysilane (APTES, 99%), N‐(3‐dimethylaminopropyl)‐N′‐ethylcarbodiimide hydrochloride (EDC hydrochloride, 99%), tetrachloroauric(III) acid trihydrate (99.9%), sodium citrate (tribasic) dihydrate, tetraethyl orthosilicate (TEOS, 98%), Pluronic F127 (BioReagent grade, 12.6 kg mol^−1^), absolute ethanol (EMPLURA, 99.5%), hydrochloric acid (HCl, 37% in water, ACS reagent grade), 3‐aminopropyldimethylmethoxysilane (APDMMS, 97%), dimethyl sulfoxide (DMSO, EMPLURA, 99%), [2‐(Methacryloyloxy)ethyl]trimethylammonium chloride (METAC) solution in water (75%, stabilized with 4‐methoxyphenol), 5,10,15,20‐Tetraphenyl‐21H,23H‐porphine zinc(II) (ZnTPP), and sodium sulfate (anhydrous, 99%) were purchased from Sigma Aldrich (St. Louis, MO, USA), Merck (Darmstadt, Germany), or TCI (Tokyo, Japan) and used without further purification unless otherwise noted. Alexa Fluor 488 (A30629) was purchased from ThermoFisher Scientific (Waltham, MA, USA). Deuterated chloroform (CDCl_3_, 99.8% atom% D) was purchased from Sigma Aldrich. Anhydrous DCM (99.8%, containing 40−150 ppm amylene) and anhydrous toluene (99.8%) were purchased from Sigma Aldrich. Unless otherwise noted, deionized water was used. Solvents not mentioned above were technical grade. Reactions sensitive toward air‐exposure were carried out under inert conditions, using standard Schlenk line techniques under nitrogen atmosphere (5.0, 99.999% purity, Linde, Dublin, Ireland).

Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectra were recorded on a Bruker DRX 500 or a Bruker AC 300 (Bruker, Billerica, MA, USA) and analyzed using the software MestreNova 14.2.3. Chemical shifts (δ) are reported relative to trimethylsilane (TMS) and calibrated on the residual peak of deuterated solvents (CDCl_3_ δ = 7.26).^[^ 67 ^]^ Coupling constants (J) are given in Hz. The following abbreviations were used for signal multiplicities: singlet (s), doublet (d), triplet (t), quartet (q), quintet (p), multiplet (m), broad (br), and combinations thereof.

UV/Vis Spectroscopy

UV/Vis spectra were recorded between 300 and 800 nm using an Agilent Cary 60 UV/Vis spectrometer (Agilent, Santa Clara, CA). All spectra were background corrected to the respective solvent. Unless otherwise noted, UV/Vis spectra were measured in glass cuvettes, using the following concentrations: 2.5 mmol L^−1^ for all iniferters and iniferter derivatives; 10 μmol L^−1^ for ZnTPP.

Attenuated Total Internal Reflection Infrared (ATR‐IR) Spectroscopy

IR spectra were recorded in attenuated total reflection (ATR) mode using a Spectrum One Fourier transformation infrared (FT‐IR) spectrometer from Perkin Elmer (Waltham, MA, USA) in the range from 4000 to 650 cm^−1^. For the measurements, performed with 10 scans per sample, the films were scratched off the substrate using a razor blade and placed on top of the ATR‐IR crystal (ZnSe) at a contact pressure of 30. The resulting spectra were background‐corrected using the device software's background correction tool and normalized to the Si—O—Si asymmetric stretching vibrational band at ≈1060 cm^−1^, or the C—N stretching vibrational band/the C—N—H deformational band at ≈1545 cm^−1^ by using Origin2023 Pro.

CO2‐Plasma Treatment

CO_2_‐Plasma Treatment of iniferter‐functionalized MPS thin films was performed with a 20 Femto plasma system (Diener Electronic, Ebhausen, Germany) analogous to Krohm et al.^[^ 59 ^]^ and Babu et al.^[^ 58 ^]^ at a pressure of 0.3 mbar and a power of 20% (10 W). The duration was 12 s.

Transition Electron Microscopy (TEM)

TEM was performed on Philips FEI CM20 (FEI Philips Electron Optics, Hillsboro, OR, USA) microscope equipped with a LAB‐6 cathode and an Olympus CCD camera (Olympus, Tokyo, Japan). The acceleration voltage was set to 200 kV for operation. MPS thin films were scratched off the substrate and suspended in ethanol using the ultrasonic bath. A drop of the ethanolic suspension was placed on a holey carbon‐coated copper grid prior to the measurement. From the recorded images, the pore sizes were determined on the basis of GV differences between the brighter void of the pore and the darker pore walls using the line profile tool of the software Fiji.^[^ 68 ^]^ The diameters of the AuNSs, part of silica‐Au composite thin films, were determined as Feret diameter from the respective TEM images using the “Analyze Particles” function of the software Fiji.^[^ 68 ^]^

GISAXS

GISAXS experiments were performed in a XEUSS 1.0 setup (XENOCS, Grenoble, France). X‐rays of 0.15419 nm wavelength were generated through a GENIX 3D micro‐focus tube. Images were recorded for incident angles between 0.15° and 0.39°. The scattered photons were detected through a PILATUS 100 K detector and placed at a sample‐to‐detector distance of 1350 mm.

Ellipsometry

Ellipsometry was employed for determination of refractive indices and thicknesses of MPS thin films deposited on silicon wafer substrates (Si‐Mat, Kaufering, Germany, 100 mm diameter, 525 ± 25 μm thickness, type P/Bor, <100> orientation, CZ growth method, 2–5 W resistivity, polished on one side) using the nanofilm EP3−SE device (Accurion, Göttingen, Germany) equipped with a 658 nm laser. The software EP4‐View and EP4‐model (version 1.2.0) were used for measurements and model analysis. The angle of incidence (AOI) was varied from 38° to 68° in 2° increments and measured in one‐zone mode (Si−wafer → SiO_2_ oxide layer → SiO_2_ mesoporous layer). The measurements were performed at 15% relative humidity adjusted by the ACEflow 1.0 (SOLGELWAY, Sceaux, France) equipment and regulated by the Regul’Hum (version 3.3) software. The refractive index (n) and layer thickness (d) of the samples were fitted through the EP4 software in up to 1000 iterations, approaching a terminal error tolerance of 10^−7^. The fits were performed within the ranges 1.0 ≤ *n *≤ 1.5 and 100 nm ≤ *d *≤ 250 nm. The calculation of porosity was carried out using the obtained average refractive indices of n = 1.250 ± 0.001 for MPS‐Au composite thin films and n = 1.216 ± 0.003 for MPS‐reference thin films according to Brüggemann's effective medium theory described in literature.^[^ 69 ^]^

Light Intensity Measurements

Light intensity measurements were performed using a PM160T Optical Power Meter (ThorLabs, Newton, NJ, USA, last calibrated 12.09.2024) with a detector area of 0.71 cm^2^. Light intensities were determined at the wavelength of maximum emission 60 s after switching on the light source. During the measurement the detector was mounted at a fixed distance to the light source.

Synthesis of P3TCMPA

P3TCMPA (2‐(propylthiocarbonothioylthio)‐2‐methylpropanoic acid) was synthesized according to the literature.^[^ 70 ^]^ At 0 °C, propane‐1‐thiol (1.52 g, 1.81 mL, 20.0 mmol, 1.00 eq.) was added to a suspension of potassium phosphate trihydrate (5.48 g, 24.0 mmol, 1.20 eq.) in acetone (60 mL) and stirred for 20 min. Afterward, carbon disulfide (2.28 g, 1.80 mL, 30.0 mmol, 1.50 eq.) was added dropwise, causing the reaction mixture to quickly turn yellow. Following an additional 60 min of stirring at room temperature, 2‐bromo‐2‐methylpropionic acid (3.29 g, 19.7 mmol, 0.99 eq) was added and stirred overnight in the dark. After removal of the solvent under reduced pressure, the yellow residue was dissolved in DCM (50 mL) and mixed with aqueous HCl (1 mol L^−1^, 50 mL). After separating the organic phase, the remaining aqueous phase was extracted twice with DCM (2 × 30 mL). The combined organic phases were dried over magnesium sulfate and the solvent was removed under reduced pressure, leading to a yellow oil which was subjected to column chromatography (R f (product) = 0.53, silica, ethyl acetate/cyclohexane 1:3°v/v). After removal of the solvent under reduced pressure, the so‐obtained yellow oil was dissolved in n‐heptane (15 mL) and stored at −25 °C for 2 h. After precipitation, the yellow crystalline product was filtered out while cold, leading to P3TCMPA in 30% yield (1.43 g, 5.99 mmol). The obtained ^1^H NMR spectrum is consistent with the literature:^[^ 70 ^]^ ^1^H NMR (500 MHz, CDCl_3_) δ = 3.27 (t, J = 7.4 Hz, 2H), 1.76–1.68 (m, 8H), 1.01 (t, J = 7.3 Hz, 3H).

Synthesis of P3TCMPA‐a1

P3TCMPA‐a1 (Figure 5) was synthesized according to the literature.^[^ 54 ^]^ In a 250 mL Schlenk flask, P3TCMPA (477 mg, 2.00 mmol, 1.00 eq.) was dissolved in anhydrous DCM (80 mL) under nitrogen atmosphere. Then, a solution of EDC hydrochloride (423 mg, 2.20 mmol; 1.10 eq.) in anhydrous DCM (25 mL) was slowly added at room temperature. After stirring for 10 min at 0 °C, APTES (443 mg, 2.00 mmol, 1.00 eq.) was added dropwise. The reaction mixture was stirred overnight, slowly warming up to room temperature. After removal of the solvent under reduced pressure, the so‐obtained crude product was purified by column chromatography (3.5 cm column diameter, silica (up to a height of 12 cm), ethyl acetate/cyclohexane 1:4 v/v), leading to P3TCMPA‐a1 in 61% yield (554 mg, 1.25 mmol).

Chemical structure of P3TCMPA‐a1; lowercase letters are assigned to the hydrogen atoms.

1

H NMR (500 MHz, CDCl

3

, δ):

6.62 (s, 1H, N—H, H^e^), 3.80 (q, J = 7.0 Hz, O—CH_2_, H^i^), 3.31–3.15 (m, 4H, NvCH_2_/S‐CH_2_/H^f^/H^c^), 1.76–1.65 (m, 8H, C—CH_3_/CH_2_, H^d^/H^b^), 1.65–1.52 (m, 2H, CH_2_, H^g^), 1.21 (t, J = 7.0 Hz, 9H, CH_3_, H^j^), 1.00 (t, J = 7.4 Hz, 3H, CH_3_, H^a^), 0.63–0.54 (m, 2H, Si—CH_2_, H^h^) ppm (Figure S11, Supporting Information).

13C NMR (75 MHz, CDCl3, δ)

220.09 (C=S), 171.86 (C=O), 58.57 (O—CH_2_), 57.39 (S—C), 42.77 (N—CH_2_), 38.95 (S—CH_2_), 26.10 (C—CH_3_), 22.81 (S—CH_2_—CH_2_), 21.48 (N—CH_2_—CH_2_), 18.45 (O—CH_2_—CH_3_), 13.62 (CH_2_—CH_3_), 7.83 ppm (Si—CH_2_) ppm (Figure S12, Supporting Information).

ESI‐MS m/z

[M+Na]^+^ calcd for C_17_H_35_NO_4_S_3_SiNa, 464.140; found 464.139 (Figure S13, Supporting Information).

Synthesis Au Nanospheres (AuNSs)

AuNSs were synthesized according to a procedure described in literature.^[^ 71 ^]^ Prior to use, a 500 mL round‐bottom flask was cleaned using boiling MilliQ water. For this purpose, ≈150 mL MilliQ water was boiled in the reaction setup for 20 min and then removed. Then, tetrachloroauric(III) acid trihydrate (34.0 mg, 43.2 μmol, 1.00 eq.) was dissolved in boiling water, keeping the temperature for 20 min under constant stirring. After cooling down for 6 min, a solution of tribasic sodium citrate dihydrate (185 mg, 315 μmol, 7.30 eq.) was added to the solution and the reaction mixture was stirred for 3 h at 80 °C in the dark. Then, the reaction vessel was removed from the heater and cooled down using an ice bath. Subsequently, the reaction mixture was filtered through a syringe filter (0.2 μm) and stored in the fridge at 4 °C. The so‐obtained reddish aqueous suspension of AuNSs was analyzed through UV/Vis spectroscopy, exhibiting a characteristic absorption maximum at 520 nm (Figure S14, Supporting Information).

MPS‐Reference Thin Films

MPS‐reference thin films were prepared through sol–gel chemistry using tetraethyl orthosilicate (TEOS) as inorganic precursor and the amphiphilic triblock copolymer Pluronic F127 as structure directing template that undergoes micellization upon solvent evaporation. According to reported protocols,^[^ 72 ^]^ the molar ratios of the sol–gel precursor solutions were set to 1 TEOS, 0.0075 F127, 40 ethanol, 10 water, 0.28 HCl. The preparation of the sol–gel precursors was performed at room temperature, first suspending Pluronic F127 in ethanol under stirring. Upon addition of a freshly prepared solution of hydrochloric acid (37%) in water (1.55 mol L^−1^), the template dissolved immediately. After the addition of TEOS, the solution was stirred overnight at room temperature and subsequently stored at −18 °C. Prior to dip‐coating, the sol–gel solutions were stirred for 30 min at room temperature and the substrates (microscopy slides (2.6 × 7.6 cm, 1000 μm thickness, float glass, VWR International, Radnor, PA, USA) or rounded cover slips (2.5 cm diameter, 170 μm thickness, borosilicate glass, VWR International, Radnor, PA, USA)) were cleaned with ethanol‐soaked fabrics. During dip‐coating, rounded cover slips were generally covered with adhesive tap on the backside. The substrates were dip‐coated at a withdrawal speed of 2 mm s^−1^ under controlled environmental conditions (50% relative humidity, 22–25 °C), using the previously reported EISA process.^[^ 57 ^]^ After aging under the controlled climate conditions for 60 min, the films were subjected to the following temperature treatment: heating up to 60 °C within 10 min and holding the temperature for 60 min, followed by a temperature increase to 130 °C within 10 min and holding the temperature for 60 min. Then the previously conducted EISA process was repeated, adding another layer of mesostructured silica to the thin films which were subjected to the following, final temperature treatment: heating up to 60 °C within 10 min and holding the temperature for 60 min, followed by a temperature increase to 130 °C within 10 min and holding the temperature for 60 min, followed by a temperature increase to 350 °C (heating rate = 1 K min^−1^) and holding the temperature for 120 min.

MPS‐Au Composite Thin Films

Silica‐Au composite films were prepared using a modified version of a synthetic procedure previously reported by our group.^[^ 40 ^]^ A first layer of mesostructured silica was deposited on glass substrates (microscopy slides (2.6 × 7.6 cm, 1000 μm thickness, float glass, VWR International, Radnor, PA, USA) or rounded cover slips (2.5 cm diameter, 170 μm thickness, borosilicate glass, VWR International, Radnor, PA, USA)) according to the preparation of MPS‐reference thin films. In all dip‐coating steps, the same sol–gel precursor composition as for the mesoporous reference films was used. After heat treatment (heating up to 60 °C within 10 min and holding the temperature for 60 min, followed by a temperature increase to 130 °C within 10 min and holding the temperature for 60 min), the mesostructured thin films were reacted with 3‐aminopropyldimethylmethoxysilane (APDMMS) while the templating agent was still present inside the pores. Up to seven film‐substrates were stacked on a PTFE sample holder and transferred to a large, flame‐dried Schlenk finger under nitrogen flow. The samples inside the Schlenk flask were dried at 250 °C again using a hot air blower while evacuated. Then, the Schlenk finger was heated to 115 °C for 45 min using an oil bad while evacuated. In another Schlenk flask, APDMMS (10 μL, 59.1 nmol) was dissolved in anhydrous toluene (100 mL). The solution was briefly purged with nitrogen for 6 min. The deoxygenated solution was transferred to the large Schlenk finger, which was heated subsequently at 80 °C for 1 h. After cooling down for 10 min, the APDMMS‐functionalized mesostructured thin films were extracted for 10 min in toluene, rinsed with ethanol and dried in air. The samples were fully incubated in the previously prepared suspension of AuNSs in water for 16 h at 4 °C in the dark, leading to the immobilization of AuNSs on the outer surface of the mesostructured thin film. After rising of the reddish film‐substrates and drying, the samples were subjected to another turn of dip‐coating, adding a second layer of mesostructured silica atop the immobilized AuNSs. After aging under the controlled climate conditions for 60 min, the films were subjected to the following temperature treatment: heating up to 60 °C within 10 min and holding the temperature for 60 min, followed by a temperature increase to 130 °C within 10 min and holding the temperature for 60 min, followed by a temperature increase to 350 °C (heating rate = 1 K min^−1^) and holding the temperature for 120 min.

Preparation of MPS‐Au‐P3TCMPA and Silica‐Ref‐P3TCMPA Thin Films

After charging a Schlenk flask with three samples (silica‐Au composite or reference films), a solution of the P3TCMPA‐a1 (23.9 mg, 54.0 μmol,) in anhydrous toluene (18 mL) was added. The reaction vessel was heated for 1 h at 80 °C in the dark. After cooling down, the substrates were extracted in toluene for 10 min, rinsed with ethanol and dried in air. Small sections of the so‐obtained MPS‐Au‐P3TCMPA thin films were immersed in DMSO and subjected to UV/Vis analysis, showing a characteristic absorption maximum at 552 nm (Figure 1d). After drying, all films were subjected to CO_2_‐plasma surface treatment.

ZnTPP‐Catalyzed Polymerizations in MPS Thin Films Induced by Widefield Irradiation

In a 50 mL Schlenk finger, an aqueous solution of METAC (75%, 11.5 g, 22.1 mmol, 1.00 eq.) and ZnTPP (1.50 mg, 2.21 μmol, 1.00·10^−4^ eq.) were dissolved in DMSO (15 mL). A 2 cm high part of the MPS‐coated glass substrate (silica‐Au‐P3TCMPA or silica‐ref‐P3TCMPA) was immersed in the reaction mix and the reaction vessel was mounted 1 cm in front of the LED source. The MPS‐coated glass substrates were irradiated for 1 h at a lamp current of 250 or 700 mA (corresponding to 14 and 69 mW cm^−1^, respectively) in the dark using an LDC590.S (590 nm) LED by Metrohm−Autolab (Utrecht, Netherlands). Upon irradiation, the samples were rinsed with water and ethanol. Then, the samples were extracted for 10 min in ethanol, for 30 min in water, for 60 min in an aqueous solution of sodium sulfate (0.5 mol L^−1^), and for 60 min in dilute aqueous hydrochloric acid (pH 3). The samples were dried in air.

Catalyst‐Free Polymerizations in MPS Thin Films Induced by Widefield Irradiations

In a 50 mL Schlenk finger, an aqueous solution of METAC (75%, 11.5 g, 22.1 mmol) was dissolved in DMSO (15 mL). The solution was purged with nitrogen for 15 min. An MPS‐coated cover slip (silica‐Au‐P3TCMPA, silica‐ref‐P3TCMPA, silica‐Au composite, or silica reference), mounted onto a stainless‐steel sample holder, was covered with an excess of reaction mixture. In quick succession, the cylindrical cavity above the MPS‐coated glass substrate was sealed with another cover slip, generating an air‐free, light‐permeable compartment above the MPS‐coated glass substrate, filled with the polymerization solution. Upon incubation for 30 min, the sample was irradiated for 5 h at 18 mW cm^−2^ in the dark, using a Prior Lumen 220 Pro metal halide lamp (Prior Scientific Instruments, Cambridge, UK), equipped with a singleband filter (FF01‐563/9‐25, Semrock, Rochester, NY, USA), limiting the emission wavelength to ≈550–570 nm, and a collimated lens which was mounted 2 cm above the sample. Upon irradiation, the samples were rinsed with water. Then, the samples were extracted for 30 min in water, for 60 min in an aqueous solution of sodium sulfate (0.5 mol L^−1^), and for 60 min in dilute aqueous hydrochloric acid (pH 3). The samples were dried in air.

Microscopy Setup for ZnTPP‐Catalyzed Laser Writing

ZnTPP‐catalyzed laser writing in MPS thin films (Figure 2) was performed using a Nikon Eclipse TI2‐E (Nikon, Tokyo, Japan) laser microscope, equipped with an N‐STORM module. The 561 nm CW laser diode (Cobolt 06‐DPL, Cobolt AB, Stockholm, Sweden) connected to the N‐STORM module was used for the irradiation of MPS thin films through a Nikon CFI Apo TIRF 60CX Oil objective. The laser operation was controlled by the software NIS‐Elements (version 5.20).

ZnTPP‐Catalyzed Laser Writing in MPS Thin Films

In a 50 mL screw‐lid jar, a solution of METAC (75% in water, 7.67 g, 14.7 mmol, 1.00 eq.) and ZnTPP (1.00 mg, 1.47 μmol, 1.00·10^−4^ eq.) in DMSO (10 mL) was prepared and subjected to an ultrasonic bath for 3 min to complete the dissolution of ZnTPP. An MPS‐coated cover slip (silica‐Au‐P3TCMPA or silica‐ref‐P3TCMPA) was mounted onto a stainless‐steel sample holder, facing upward with the MPS‐coated side. The charged sample holder was fixed on the sample stage of the Nikon Eclipse TI2‐E laser microscope. The substrate was covered with an excess of the previously prepared reaction solution and left for incubation for 30 min in the dark. Prior to laser writing, immersion oil was applied to the 60× objective which was then elevated toward the substrate, focusing on the surface of the MPS thin film. Under protection from external light, an array of positions was irradiated with 561 nm laser under controlled irradiation times and powers using a beam expander setting of “8×” and a 60× immersion objective. The targeted arrays of positions were encoded in a list of x/y coordinates using the built‐in macrofunction of the operating software which directed the motorized sample stage while opening and closing the microscope's shutter as well as controlling the irradiation powers. Upon irradiation, the samples were rinsed with water and ethanol. Then, the samples were extracted for 30 min in water, for 60 min in an aqueous solution of sodium sulfate (0.5 mol L^−1^), and for 60 min in dilute aqueous hydrochloric acid (pH 3). The samples were dried in air.

Microscopy Setup for Uncatalyzed Laser Writing

Uncatalyzed laser writing in MPS thin films (Figure 3 and 4) was performed using custom‐built laser microscope: The output of a 561 nm CW laser diode (Obis LS 561, Coherent Corp., Saxonburg, PA, USA) was coupled into a single mode fiber (kineFLEX, Qioptiq, Excelitas Technologies Corp., Waltham, MA, USA), and filtered by a quadband excitation filter (FF01–563, Semrock, Rochester, NY, USA). The 561 nm laser was used for the irradiation of MPS thin films through a CFI Apo TIRF 60× Oil objective (Nikon, Tokyo, Japan) or a CFI Apo TIRF 100× Oil objective (Nikon, Tokyo, Japan). Objectives and filters were mounted in a Nikon TI‐E microscope body (Nikon, Tokyo, Japan). The laser operation was controlled by software.

Uncatalyzed Laser Writing in MPS Thin Films

In a 50 mL Schlenk finger, an aqueous solution of 4‐methoxyphenol stabilized METAC (75%, 11.5 g, 22.1 mmol) was dissolved in DMSO (15 mL). The solution was purged with nitrogen for 15 min. An MPS‐coated cover slip (silica‐Au‐P3TCMPA or silica‐ref‐P3TCMPA) was mounted onto a stainless‐steel sample holder, facing upward with the MPS‐coated side. The charged sample holder was fixed on the sample stage of the custom‐built laser microscope and covered with an excess of the reaction solution. In quick succession, the cylindrical cavity above the MPS‐coated glass substrate was sealed with another cover slip substrate, generating an air‐free, light‐permeable compartment above the MPS‐coated glass substrate, filled with the polymerization solution. Prior to laser writing, immersion oil was applied to the 60× or 100× objective which was then elevated toward the substrate, focusing on the surface of the MPS thin film. The angle of the incident laser beam was set to 0°, hence directing the laser beam perpendicular to the sample plane. Upon incubation for 30 min in the dark, an array of positions was irradiated with a 561 nm laser under controlled irradiation times and powers using 60× or 100× immersion objective. The laser operating software controlled the irradiation times/powers and directed the motorized sample stage (interspot distance was set to 150 μm) while opening and closing the microscope's shutter. Upon irradiation, the samples were rinsed with water three times. Then the samples were extracted in water for 30 min and dried in air.

Fluorescence Microscopy

According to the literature,^[^ 30, 73 ^]^ the PMETAC‐functionalized samples, obtained from laser writing, were incubated in a solution of Alexa 488 in water (1 μg mL^−1^) for 10 min. Then, the samples were extracted for 30 min in water and subsequently subjected to fluorescence microscopy:

Figure 2

The related microscopy images were recorded by epi‐fluorescence imaging, using a 10× Nikon CFI Plan‐Fluor objective (numerical aperture (NA) = 0.3; working distance (WD) = 15.2 mm; achievable resolution = 1.07 μm) and fluorescent lamp (SOLA Light Engine, Lumencor, Beaverton, OR, USA) as part of the TI2‐E microscopy setup for widefield irradiation of the samples at an excitation wavelength of 488 nm. During all microscopy experiments, a constant irradiation output power of 1.9 mW cm^−2^ (software setting: 100) was maintained using the NIS‐Elements software (version 5.20). The exposure time was set to 60 ms per image, and images were acquired with 4 × 4 pixel binning. The images were recorded using an Andor ZYLA 4.2 PLUS camera (Andor Technologies, Belfast, UK) while tracking the emission wavelength in the range of 500–550 nm using a band‐pass filter (Chroma Laser Filter set F46‐103, AHF analysentechnik AG, Tübingen, Germany).

Figure 3

The related microscopy images were recorded using a TCS SP8 confocal microscope (Leica, Wetzlar, Germany), equipped with a 20× HC PL APO 20×/0,75 CS2 air objective by Leica (NA = 0.75; WD = 0.62 mm; achievable resolution = 0.43 μm) and a HyD detector. The samples were irradiated with a 488 nm laser, tracking the fluorophore emission in a wavelength range between 493 and 707 nm.

Figure 4

The related microscopy images were recorded by epi‐fluorescence imaging, using a 20× Nikon CFI Plan Apo Lambda air objective (NA = 0.75; WD = 1 mm; achievable resolution = 0.43 μm) and a Prior Lumen 220 Pro white light metal halide lamp (Prior Scientific Instruments, Cambridge, UK) equipped with an FF02–482/18–25 filter (Semrock, Rochester, NY, USA), for fluorophore excitation in a 473–491 nm wavelength range. During all microscopy experiments, a constant irradiation output power of 6.7 mW cm^−2^ (software setting: 100) was maintained using a custom‐built software. The exposure time was set to 60 ms per image. The incident light was coupled into the microscope via a liquid light guide using the second of two of the Nikon Microscope's “stratum structure” beam path. Fluorophore emission was imaged onto an iXon EM + DU‐897 EMCCD camera (Andor, Belfast, UK), using a FF01–446/523/600/677–25 quadband emission filter (Semrock, Rochester, NY, USA) to track the emission wavelength in the range of 502.5–544.5 nm.

Determination of the GV of Polymer Spots

For the determination of GVs of fluorophore‐colorized PMETAC polymer spots images, recorded by fluorescence microscopy, were evaluated using the software Fiji.^[^ 68 ^]^ After retrieval of the polymer spots, circles of fixed area were created and centered on the middle point of the selected polymer spots following an automated protocol (Figure S10, Supporting Information). Then, the GVs, averaged over the circles’ area, were determined using the software. To enable the comparability of the GVs of different samples, the measured gray values (GV_0_) were background‐referenced to the background gray value (GV_bg_), excluding the polymer spots, to obtain the intended background‐referenced gray values (GV_ref_) (Equation 1)

$$[eqn]$$

In correlation with an increased pore order, increased averaged GV_bg_ values by a factor of 1.03 (silica‐ref‐P3TCMPA: silica‐Au‐P3TCMPA) were measured for silica‐ref‐P3TCMPA films. GV_bg_ excludes the polymer spots 1 to n, characterized through their specific gray values GV_i_ and areas A _ i , and was calculated from the gray value of the entire microscopy image (GV_avg), averaged over the area of the image (A image) (Equation 2)

$$[eqn]$$

For the uniform‐sized microscopy images of Figure 4, GV_avg_ was calculated for a predefined polygon. For the 512 × 512 pixel images, the polygon was implemented through the following imagej command: makePolygon(1,70,65,2,425,3,509,78,511,439,431,510,53,510,3,443).

Determination of the Width of Polymer Spots

Unless otherwise noted, the widths of polymer spots were calculated from the GV profile of the respective spots along their transverse section. Using the software OriginPro 2023, the GV profiles were modeled by Gaussian function according to Equation (3)

$$[eqn]$$

The width of the polymer spots was defined as difference between the two x values x 1 and x 2 of the Gaussian function with a GV of 13.5% (1/e ^2^) of the maximum GV at GV(x c). Thus, x 1 and x 2 correspond to the GV in Equation (4)

$$[eqn]$$

The width b of the polymer spots is given by Equation (5)

$$[eqn]$$

To obtain x 1 and x 2, the Equation (3) and (4) were equated and solved for x, leading to the following expressions for x 1 and x 2 (Equation 6)

$$[eqn]$$

If applicable, spot widths were averaged over all polymer spots of the respective microscopy image, irradiated under equivalent conditions.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supplementary Material