STATEMENT OF GOVERNMENTAL INTEREST

[0001] This invention was made with government support under FA9550-16-1 -0150 awarded by the Air Force Office of Scientific Research and DB11353682 awarded by the National Science Foundation. The government has certain rights in the invention.

BACKGROUND

[0002] Photochemical patterning has shown its ability for spatial and temporal control of the chemical composition of materials surface at the nanoscale. ^1"3 In particular, numerous photoreactions are conducted in the solution phase, such as photo-click reactions ^4"5 and photopolymerizations. ⁶ They allow the surface functionalization with diverse molecules, biomacromolecules and nanomaterials as well as the construction of complicated structures, which have found wide applications in research and

manufacturing, such as wettability control, ^7"8 printed electronics, ^9"10 bio-chips, ^11"13 3D & 4D printing, ^14"17 cell-material interface studies, ^18"20 etc. Currently, there are two common approaches for performing the surface photochemistry in a site-selective manner:

parallel tools including the mask-based photolithography and the mask-free digital- mirror-device (DMD) projection technique, and serial writing based on laser scanning. While the parallel approaches afford wafer-scale patterning, the feature size is virtually limited to a few micrometers. Phase-shift ^21"22 or interference ²³ photolithography can push the resolution beyond the diffraction limit, but they only allow fixed patterns to be produced. Laser scanning, on the other hand, is an arbitrary writing technique with the resolution down to -100 nm by the utilization of two-photon absorption mechanism. ²⁴ However, to meet the sub-micrometer resolution, it is often accompanied with sophisticated instrument and small patterning area (less than 1 mm ² ). Thus, a need exists for a lithography tool and method that can print large patterning areas with high resolution, high speed, and flexibility to generate desired patterns.

SUMMARY

[0003] Provided herein are tip arrays comprising a plurality of tips fixed to a common substrate layer and an optional support layer, the tips formed from a high-refractive-index polymer, said high-refractive-index polymer having a refractive index of 1 .65 or greater, and each tip coated with a metal layer having a thickness of about 50 nm to about 250 nm, optionally the metal layer positioned on each tip to leave an aperture at one end of the tip.

[0004] In various cases, the high-refractive-index polymer has a refractive index of 1 .65 to 2. In various cases, the high-refractive-index polymer comprises SU-8 or NOA 170. In various cases, the metal layer comprises gold. In various cases, the metal layer is arranged on each tip to leave an aperture at one end of the tip. In various cases, the tip array comprises a common substrate layer. In some cases, the common substrate comprises an elastomer, and in some cases, the common substrate comprises a mixture of

polydimethylsiloxane oligomers and crosslinkers (e.g., Sylgard® 184).

[0005] Also provided herein are methods for printing a photosensitive surface comprising positioning the tip array described herein on or near the photosensitive surface, irradiating at least one tip of the tip array with a radiation source to transmit radiation through the tip to the photosensitive surface to print indicia on the photosensitive surface, and optionally moving the photosensitive surface, the tip array, or both and repeating the irradiating step. In various cases, the photosensitive surface comprises a self-assembled monolayer or hydrogel on a gold surface. In various cases, the tip array and substrate surface form a gap during the irradiating step. In various cases, a solvent of refractive index less than 1 .35 is present at the tip array and photosensitive surface. In some cases, the solvent comprises water, methanol, or acetonitrile.

[0006] Also provided herein are methods of making tip arrays as described herein comprising providing a mold comprising an array of recesses, optionally coating the recesses with a metal layer, applying a high-refractive-index polymer to the mold to the fill the recesses, curing the high-refractive-index polymer, casting a common substrate over the mold, optionally applying a support layer on top of the common substrate, and separating the tip array from the mold. In various cases, the coating of the recesses with the metal layer comprises a tilted rotational evaporation process such that the recesses are not coated with the metal layer at the tips and form the apertures. In various cases, the coating of the recesses with the metal layer comprises coating the metal layer over the entirety of the recesses and the metal layer at the tips of the tip array are removed to form the apertures after separating the tip array from the mold. In various cases, the metal layer comprises gold. In various cases, the curing comprises irradiation with UV light.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1 shows (A) a schematic of photochemical desorption of thiol self- assembled monolayers followed by Au etching. PI: photoinitiator; (B) finite-difference time- domain (FDTD) simulations for light propagation through a fully-metal-coated SU-8 tip (100- nm Au) and a metal-free SU-8 tip in the aqueous environment (scale bars: 500 nm); and (C) a plot of the simulated light intensities normalized to the incident light with the propagation length through the tip apex.

[0008] Figure 2 shows (A) a schematic of the fabrication process for SU-8 and NOA tip arrays: (a) spin coating of SU-8 photoresist onto a 100-nm Au-coated Si master followed by UV curing; (b) sandwiching the PDMS liquid precursor between the UV-cured SU-8 layer and a glass slide followed by PDMS curing at room temperature for 72 h; (c) peeling off the whole assembly from the Si master; (d) spin coating of PMMA layer onto the Au-coated pyramid arrays, (e) fabrication of micro-sized apertures by etching the uncovered Au on the tips; (B) a scanning electron microscopy (SEM) image of the 100-nm Au-coated SU-8 tip; (C) an SEM image of the micro-apertured SU-8 pyramid with a metal-free tip; and (D) SEM images of the SU-8 pyramid arrays with uniform micro-apertures.

[0009] Figure 3 shows (A) a dark-field optical microscope image of typical Au patterns generated with the fully-metal-coated SU-8 tips; (B) an AFM image of sub-wavelength Au features patterned under the conditions that the tip was in contact with and lifted away from the substrate; and (C) dark-field optical microscope images of large-area nanopatterns with the tip-substrate gap less than 1 μηι, where from left to right the tip height was gradually decreased.

[0010] Figure 4 shows (A) an optical microscopy image of Au patterns written by the micro-apertured SU-8 pen array, with the size of metal-free tips in the range of 1 .5-3 μηι;(Β) an atomic force microscopy (AFM) images of a typical array produced by a 1 .5-μηι SU-8 tip; (C) an AFM image of sub-200 nm feature produced by the metal-free tip; (D) a plot of feature area with the exposure time for metal-free tips with 1 .5-μηι tip size in average; (E) SEM images of Au patterns generated by metal-free tips with varied sizes; (F) a plot of the feature diameter with the exposure time for different tips; and (G) a plot of the growth rate of feature diameter with the size of metal-free tip.

[0011] Figure 5 shows (A) depicts merged optical microscope images of etched Au patterns over a 3x3 mm ² area; (B) a schematic of the exchange of thiolated oligonucleotides followed by the DNA-directed nanoparticle assembly; (C) an optical microscopy image of the resulting Au nanoparticle patterns; (D) an SEM image of the site-selectively assembled nanoparticles with feature size ranging from -500 nm to ~2 μηι under varied exposure times; and (E) photopatterning of hydrogels with thiol-ene photochemistry, where the fluorescent microscope image shows the attachment of Rhodamine-labeled thiols onto the PEG hydrogel surface.

[0012] Figure 6 shows (A and B) simulations of light transmission through pyramidal tips in water for apertureless PDMS tips, SU-8 tips with 40-nm and 100-nm Au coating, and SU-8 tip with a 400-nm aperture; and (C) a UV-Vis transmittance spectrum of the glass substrate coated with SU-8/PDMS thin layers.

[0013] Figure 7 shows (A) generation of arrays of etched Au holes through the

photochemical desorption in the photoinitiator solution using a photomask and a low-power 405-nm laser pointer; and (B) an X-ray photoelectron spectroscopy (XPS) characterization of the dodecanethiol SAM-coated substrate before and after exposure to a 405-nm LED under 34 mM LAP solution. [0014] Figure 8 shows (A) an optical microscope image of arrays of sub-micrometer Au features over large areas, with exposure times of 5-20 s, with the insert showing a histogram of the feature sizes at 1 5 s of exposure across a -2 x2 mm ² patterning area; (B) an AFM image of a typical array produced by a single tip, with exposure times of 5, 1 0, 1 5 and 20 s from top to bottom; and (C) the average feature size in 5 regions over a -2x2 mm ² patterning area.

DETAILED DESCRIPTION

[0015] By combining DMD projection with scanning probe lithography, beam pen lithography (BPL) has offered a platform for photopatterning in a massively multiplexed and direct-write fashion with an inexpensive desktop instrument. ^25"27 It operates millions of polymer tip arrays to direct light onto a surface through the nanoscopic apertures in each tip apex. Moreover, individual pen addressability has been realized to produce complicated patterns on the photoresists with 1 00-nm features over 1 cm ² area. ²⁶

[0016] Nevertheless, when conventional polydimethylsiloxane (PDMS) beam pen arrays are applied to perform photoreactions in solution, there are several challenges leading to either low throughput or low resolution, which hinders its wide application in the chemical and biological contexts. First, multiple and complicated steps are needed to fabricate the sub-wavelength apertures, and uniform apertures are difficult to produce with consistency. Second, photopatterning using the nano-apertures could suffer from slow writing speed, because of the low transmission efficiency (< 1 0%) of light and fast power dissipation at the sub-wavelength distance when the light passes through a tiny hole smaller than the incident light wavelength. ^28"29 The situation could become worse in the solution-phase

photoreactions, which generally require >1 00 times higher optical energy than photoresists. Thus, it could take hours or even days to write a complicated pattern. Improvement was realized in the lithography with a single scanning near-field probe, where short-wavelength lights (<300 nm) or high-power laser sources are employed to increase the speed. However, the patterning areas remained limited to microscales. ³ Such instrumental improvement cannot be easily incorporated with BPL, since the polymer-based tip arrays as well as the DMD projection system can manipulate the light only with longer wavelength (>350 nm) and moderate power (<2 W cm ^"2 for UV light). In addition, the high-power light sources could limit the illumination area and increase the cost and safety risk. Additionally, maintaining a sub-wavelength tip-substrate gap for each pen is challenging during the solution-phase patterning, which would not only increase the feature sizes but also reduce the pattern uniformity over large areas.

[0017] To address these challenges, disclosed herein is a general materials-based approach that allows for high-speed solution-phase photochemical patterning under mild conditions while maintaining the sub-wavelength feature sizes and macroscopic patterning area. The innovation is rendered by engineering the optical properties of tips made of high- refractive-index polymer material (having a refractive index of 1 .65 or greater) to realize light focusing at the tip apex in the liquid media free of the nano-apertures. Massive tip arrays with different tip configurations can be fabricated, including (1 ) fully-metal-coated and (2) metal-free tips.

[0018] Tip-directed surface free-radical photoreactions were systematically investigated by utilizing the photochemical desorption of self-assembled monolayers (SAMs) of thiolates on the Au surface in the presence of an aqueous photoinitiator solution (Figure 1 A). This photochemical nanopatterning approach was applied for rapid generation of large-area arbitrary metal and molecular patterns, which showed proof-of-concept demonstrations for creating platforms that are interested in many chemical, materials and biological studies.

[0019] Previous studies on scanning near-field microscopy and beam pen lithography have shown that, in air, apertureless pyramidal tips are capable of directing light onto the surface with the performance similar to the sub-wavelength apertures. Two types of apertureless configurations were introduced before: the tip fully coated with an opaque metal layer ³⁰ and the tip without metal coating ³¹ . In both cases, in order to render the light being predominately focused at the apex, a high degree of refractive index difference between the pyramidal tip and the surrounding environment (air) is needed. Therefore, a refractive index of 1 .65 for the tip material is required for light focusing at the apex in a liquid medium with n<1 .34, which corresponds to common solvents such as water, acetonitrile and methanol. Among those commercially available polymers, SU-8, an epoxy-based negative tone photoresist, meets the optical requirement well (n>1 .65 for λ<400 nm, >50% transmittance for λ>350 nm). ³² Moreover, it has been utilized as a suitable material for making plastic scanning probes with a simple molding process. ³³ Contemplated polymers having refractive indices of 1 .65 and greater include SU-8 (1 .65), poly(p-phenylene ether-sulfone) (1 .6500), poly[diphenylmethane bis(4-phenyl)carbonate] (1 .6539), polyvinyl phenyl sulfide) (1 .6568), poly(styrene sulfide) (1 .6568), butylphenol formaldehyde resin (1 .6600), poly(p-xylylene) (1 .6690), poly(2-vinylnaphthalene) (1 .6818), poly(N-vinyl carbazole) (1 .6830), naphthalene- formaldehyde rubber (1 .6960), phenol-formaldehyde resin (1 .7000), poly(pentabromophenyl methacrylate) (1 .7100), NOA 170 (1 .70), and organic-inorganic composites comprising a polymer matrix and a metal oxide nanoparticle (e.g., Ti0 ₂ , ZnO, Zr0 ₂ , and Ce0 ₂ ) (1 .66- 1 .993) (see, e.g., ref. 34). A polymer with a refractive index of 1 .65 or greater can be used to prepare the tip array as disclosed herein. While SU-8 and NOA170 are specifically used in the examples and discussion herein, it is understood that other polymers having a refractive index of 1 .65 or greater can be substituted. [0020] To explore the optics of the SU-8 tips in the solution, finite-difference time-domain (FDTD) calculations of light transmission through the SU-8 tips in water (n=1 .34) were conducted. Two types of 1 .5^m-sized tips were investigated, each coated with 100-nm Au and without Au, respectively (Figure 1 B). As expected, light can be directed to the vicinity of the tip and propagates in the liquid media with little lateral spreading by both the

apertureless configurations. Consequently, focused light beams with diameters of about 300 nm can be formed underneath the tip along a depth larger than the light wavelength. In contrast, conventional PDMS tips with the same tip configurations fail to focus the light (Figure 6A). Simulations also suggest that, compared with a 40-nm Au coating (Figure 6B), the 100-nm Au coating on the SU-8 tip allows less light leakage from the tip to the surrounding space to maintain a round light spot. More importantly, after examining the light intensity transmitted through the tips, the metal-free tip was found to result in a remarkable intensity enhancement and a longer propagation length. The normalized light intensities (the incident light=1 ) with the propagation length from the tip apex are shown in Figure 1 C. With the metal-free tip, the focused light at the tip apex exhibits an intensity of >20 fold higher than the incident light, and no intensity decay was observed within the initial 0.5-μηι propagation. Traveling beyond 0.5 μηι, the light intensity starts to decrease by a factor of ~5 per 1 μηι. For the fully-metal-coated tip, the transmitted light intensity is about 30% of the incident light at the tip apex and exhibits exponential decay from the tip apex by a factor of about 10 per 1 μηι. But it still shows longer penetration depth with less lateral spreading in comparison with an SU-8 probe with a 400-nm aperture (Figure 6B). Thus, massive arrays of the SU-8 tips are promising candidates to direct light at the sub-wavelength scale in the solution without the need for sub-wavelength apertures. Furthermore, the metal-free tip could significantly improve the throughput by focusing the light more intensely with confined propagation beyond the near-field.

[0021] Apertureless SU-8 tip arrays were fabricated by employing a template-stripping method (Figure 2A, method 1 ). ³³ Briefly, a hydrophobic silicon mold used for making conventional polymer pen arrays (see, e.g., WO 09/132321 ) was first coated with a 100-nm- thick layer of Au by evaporation, which served as both the separation layer and the opaque coating on the final tip arrays. The Au coating can be 50 nm to 250 nm, 75 nm to 150 nm, or about 100 nm. Other metal coatings can be used instead of gold, such as silver, aluminum, or chromium.

[0022] Next, an SU-8 layer with thickness of a few tens of micrometers was spin-coated and UV-exposed on the metal-coated mold, followed by the addition of a PDMS soft backing layer and a glass support. After peeling off the whole from the mold, the obtained full-metal- coated tip arrays presented smooth metal coating and 100-nm-sized tip apex, as seen from the SEM images in Figure 2B. Furthermore, by utilizing the previously reported procedures, ³¹ uniform arrays of micro-apertures on the pyramids can be produced (40 μηι base and 100 μηι pitch), with a sharp tip radius of about 70 nm for the metal-free tips (Figures 2C and 2D). The size of the metal-free tips, that is, the edge length of the micro- apertures, can be controlled from one to tens of micrometers. The light transmittance through the pen array after removing all the metal coatings was measured as about 50% at 365 nm and >80% for λ>400 nm (Figure 6C).

[0023] NOA 170 tip arrays can be generated using the same procedure above or a resist- free method to open micro-apertures (Figure 2A, method 2). The second method comprises a tilted rotational evaporation process to produce the metal-coated silicon master with the pyramid bottom free of metal. ³⁵ Then, NOA 170 was poured onto the master, sandwiched with the glass support, and UV-cured. The addition of a soft backing elastomer is optional. Finally the whole assembly was peeled off to obtain the micro-apertured pen array. The second method is not suitable for polymers with high Young's modulus (>1 GPa) such as SU-8, which will fail to be separated from the silicon master.

[0024] To investigate the tip-directed photochemistry in the solution phase, the desorption of self-assembled monolayers (SAMs) of thiolates on the Au surface mediated by photo- generated free radicals in the presence of a photoinitiator solution was exploited. It is contemplated that the tip arrays can be used to induce other photochemical reactions or transformations, and desorption of SAMs of thiolates on gold surface is used as a proof-of- principle. Thiol SAMs have been reported to be easily desorbed from the Au surface upon irradiation due to the oxidation of surface-bound thiols to solvent-labile species by the produced reactive oxygen species. ³⁴ Thus the patterning results can be immediately visualized by etching the unprotected Au and quantify the tip-induced surface

photoreactions. Additionally, the short lifetime of free radicals results in <100 nm diffusion length, which can enable highly localized surface reactions at the nanoscale. As a primary test to show the effectiveness of the photochemistry, an Au substrate modified with 1 - dodecanethiol or (1 1 -mercaptoundecyl) tri(ethylene glycol) (EG ₃ ) was covered with an aqueous solution containing 1 % (w/v, 34 mM) photoinitiator lithium phenyl(2,4,6- trimethylbenzoyl)phosphinate (LAP), followed by exposure to the UV light through a photomask. Au patterns of etched holes can be readily generated as exposed to 365 nm light for just 40 s (about 0.25 W cm ^"2 , about 10 J cm ^"2 ) or under 405 nm (about 0.15 W cm ^"2 ) with a dosage of about 50 J cm ^"2 (Figure 7A). Additional X-ray photoelectron spectroscopy (XPS) characterization showed a decrease in both C1 s and S2p peaks (Figure 7B), but no significant C-0 bonds appeared, indicating that the Au etching can be attributed to the breakage of Au-S bonds followed by the desorption of SAMs. In the following lithography experiments, the EG ₃ SAMs were employed by taking advantage of its easily wettable and bio-repelling surface. [0025] As the first proof-of-concept demonstration of the tip-directed photochemistry, the photochemical patterning performance of the fully-metal-coated pen arrays were examined with an area of about 2 x 2 mm ² (about 400 pens). Pen arrays of different sizes are also contemplated, for example with pens (also referred to as tips) of 500 or greater, 1000 or greater, 2000 or greater, or up to 10 million. The pen array was mounted onto a scanning probe system (XE-150, Park Systems) and leveled with respect to the EG ₃ SAM-coated Au substrate optically. ³⁵ After injecting the LAP aqueous solution (34 mM) between the tip array and the substrate, the tips were then programmed to write dot arrays, in which the exposure time was set as 60 s to 180 s and the z-piezo was moved with a 0.5 μηι step. During patterning, UV light (365 nm, about 0.25 W cm ^"2 ) was applied onto the backside of the tip array simultaneously to initiate the photoreactions. As a control, dot arrays without UV illumination were also written to determine the effect of the tip contact on the SAMs. Once the patterning was finished, the Au substrate was rinsed and immersed in an aqueous solution containing 20 mM thiourea, 30 mM iron nitrate, 20 mM hydrochloric acid, and 2 mM octanol to etch the unprotected Au. Notably, the control patterns written in dark were not seen except for some approaching dots under large applied force, indicating that the bare tip contact has negligible damage to the SAMs. Under the UV, however, etched Au patterns with sub-micrometer sizes appeared until the exposure time reached 120 s and the tip was proximately close to the substrate, for instance, with a gap less than 1 μηι (Figures 3A and 3C). This is in consistent with the simulated light transmission and the drastic intensity decay as the light propagates through the metal-coated tip. Using atomic force microscopy (AFM) characterization, it was discovered that features as small as about 400 nm by about 200 nm could be produced (Figure 3B) with a sub-micrometer tip-substrate gap, and a shallower ring-like feature was observed when the tip was further extended. These phenomena not only prove the light focusing effect of the tip, but also indicate that the surface photoreactions are spatially confined at the nanoscale. Without being bound by theory, it is thought that when the tip is contacted with the substrate during exposure, free radicals could be generated only around the tip, not in the contacted area. Thus, the ring features are formed with the size determined by the tip deformation. Although in this experiment, only about half the tips wrote the patterns successfully due to the limitations in optical leveling, the implement of force-feedback leveling techniques would significantly improve the large-area patterning uniformity. ^36"37

[0026] The performance of the metal-free SU-8 tips, which are expected to enhance the propagation of light, was then evaluated. A roughly 2 x 2 mm ² micro-apertured SU-8 pyramid array was utilized, in which the side length of the metal-free pyramidal tips ranged from 1 .5 to 3 μηι. Notably, when the tips were immersed in the aqueous solution, a significant difference in the optics of the naked SU-8 and PDMS tips used for leveling was observed. The SU-8 tips maintained the same clear contrast in the pyramidal shapes as that seen in air, while blurred tips were seen in the PDMS ones. This is a direct evidence for the self- light-focusing capability of the metal-free SU-8 pyramidal tip in water, and a great benefit for optical leveling in the liquid. The lithography experiments were conducted following the same procedures above (34 mM LAP, 365 nm light, about 0.1 W cm ^"2 ). Two rows of dots were produced by each tip, in which the top row was patterned as the tip was elevated 1 μηι away from the substrate compared with the bottom row. Within each raw, the exposure time was set as 20 s to 80 s, respectively. After photopatterning, almost all tips were found to generate patterns over the entire area, and a portion of the patterning area is shown in the optical microscope image (Figure 4A). A typical array produced by the 1 .5-μηι metal-free tip was characterized by AFM and shown in Figure 4B. Sub-wavelength features were generated at 20 s and features as small as 190 nm could be achieved when patterning at the relative z=1 μηι (Figure 4C). However, in these small features, the Au layer was not etched through. With increasing exposure time, the whole 50-nm-thick Au layer was etched and the feature size increased to about 1400 nm. Au patterns were also produced using 405 nm light (about 0.15 W cm ^"2 ) with an exposure time as fast as 30 s. Based on the dosage necessary to generate Au patterns and the exposure time, the light intensity from the metal- free tip in the experiments was estimated to have an enhance factor of about 10 compared with the incident light. This result is in agreement with simulations that predict greater than 10-fold intensity enhancement within the 1 -μηι propagation length. To generate sub- micrometer patterns with similar sizes, the exposure time was approximately doubled when the tip was lifted by 1 μηι, in accordance with the light intensity at the 1 -μηι propagation length being reduced to half of that at the tip apex in the simulation as well. To further examine the rate of the surface photoreactions, the area of each feature was then plotted with the exposure time (Figure 4D). It was shown that by lifting the tip to vary the light intensity, the growth of the feature area exhibited distinct behaviors. At lower light intensity (z=1 μηι), the feature area increases slowly at the early stage (before 40 s), and gradually accelerates at longer exposure time. Under higher intensity (z=0 μηι), the feature area shows a linear increase with the exposure time, and reaches a plateau after 60 s. At 80 s, the size of the features generated at z=1 μηι slightly exceeds those generated at z=0 μηι, which can be caused by the far-field diffraction of light. Interestingly, the average rates were similar for the fast growth range at z=1 μηι and the linear growth range at z=0 μηι, suggesting that there could be three stages in the surface reactions. At the early stage of the reactions, the SAMs are not completely damaged by the photoinitiators, leading to partial etching of the Au and slow increase of the feature size. After a certain time, a large portion of SAMs can be removed to allow the full etching of Au. The feature growth rate herein can be determined by the spatial distribution of the light beam as well as the equilibrium between the photodecomposition and the diffusion of photointiators. Finally, at a longer reaction period, the growth rate could drop down and form the curve with the plateau due to the photoinitiator consumption at the local illuminated areas.

[0027] Another factor that can affect the patterning performance is the size of the metal- free tips, since the amount of transmitted light is proportional to the aperture area. Patterning from several metal-free SU-8 tips within a small region on the tip array was investigated, in which the tip size varied from 1 .7 μηι to 5.3 μηι (34 mM LAP, light intensity about 0.1 W cm ^"2 , exposure time 10-60 s). As expected, the larger tip produced Au patterns with larger sizes, as shown in the SEM images of Figure 4E. Without being bound by theory, it is thought that the larger the metal-free portion on the pyramidal tips, the more light can be focused to the tip. In this experiment, three-stage growth was observed for the larger tips (Figure 4F), consistent with the above discussions. Within a certain range, the feature diameter exhibited first-order function with the exposure time for all the tips. The growth rates in these linear ranges were found to increase dramatically when the tip size is larger than 3.5 μηι (Figure 4G). Without being bound by theory, it is hypothesized that this could result from the nonlinear increase of the radical generation rate with varied light intensities.

[0028] Not only can nanoscale free-radical reactions be studied by the SU-8 tips, but massive tip arrays in combination with the rapid photochemistry pave the way to generate nanopatterns of various materials and multi-scale surface chemical libraries with high throughput. By using arrays of tips over a 1 cm ² area, more than 1 million etched Au hole patterns can be produced with the size ranging from about 500 nm to about 3 μηι on the entire area. In Figure 5A, a patterning area of 3x3 mm ² was shown with good uniformity. The size variation over large areas could be controlled to less than 20%, which is affected by the variations in tip-substance distance (<1 μηι), aperture size (<10%), and light distribution (<10%). For example, the size distribution of sub-micrometer features patterned under the same exposure time (5 s) was measured, which was 650 ± 150 nm across the 3x3 mm ² . Notably, each feature can be patterned as fast as 2 s using 365 nm light (about 0.25 W cm ^"2 , 34 mM LAP) and the patterning process can be finished within 15 min. Moreover, sub- wavelength patterning can be also performed with photoinitiators dissolved in methanol, such as bis(2,4,6-trimethylbenzoyl)-phenylphosphineoxide (Ciba ^® IRGACURE ^® 819). The patterning speed can be further increased, especially under light with λ>400 nm, due to the higher absorption efficiency of the photoinitiators. Alternatively, as a proof-of-concept demonstration for generating molecular patterns by UV-promoted exchange, the substrate upon site-selective illumination was immersed in the solution containing thiolated

oligonucleotides to create DNA arrays. The patterns were made optically visible by further DNA-directed assembly of gold nanoparticles, with dot and line arrays of gold nanoparticles being produced (Figures 5B and 5C). ³⁸ Compared with the Au etching experiments, the patterns yielded at the same exposure condition were much larger, indicating the molecular exchange could occur at lower extent of photoreactions than the Au etching. ³⁹ By adjusting the exposure time, high-resolution patterns of the assembled nanoparticles could still be achieved, with the dot size as small as about 500 nm and the line width approaching 1 μηι (Figure 5D). For the line patterns, by increasing the line scanning speed, the density of the anchored nanoparticles was lowered in addition to the decrease in the line width. Thus, the surface density of molecular patterns can be tuned by varying the scanning speed. In addition, the tip arrays were exploited to pattern the surface of hydrogels by thiol-ene click reactions. Photocrosslinked hydrogels made from poly(ethylene glycol) diacrylate (PEGDA, average Mn 700) were immersed in an aqueous solution added with Rhodamine-labeled thiol molecules and LAP photoinitiators. Upon exposure to 365 nm light (-10 mW cm ^"2 ) through the SU-8 tip array, fluorescent patterns were obtained with irradiation for 1 min (Figure 5E). These results demonstrated the great potential of the SU-8 tip arrays for high- resolution photochemical patterning compatible with various chemical and biological environments.

[0029] In conclusion, high-refractive-index pyramidal tip arrays were introduced and evaluated to confine light and perform photochemical patterning in the solution at the about 200 nm scale free of the sub-wavelength apertures. More importantly, high throughput can be achieved with the metal-free tips, which can enhance the light intensity by two orders of magnitude compared with the fully-metal-coated or nano-apertured tips over a long propagation length in the solution. Therefore, rapid and uniform nanopatterning over cm ² areas can be easily realized using low-power and long-wavelength light sources. This simple and maskless tool provides unique approaches to light-directed synthesis within micro- /nano-scales and surface functionalization in high efficiency for applications with relevance to chemistry, biology and materials science. Furthermore, in combination with the DMD projection and microfluidics to allow multiplexed patterning capability, ^{26, 40"41} complicated surface structures of a wide diversity of chemical compositions can be envisioned for high- throughput combinatorial screening.

High refractive index tip arrays

[0030] Provided herein are tip arrays including a plurality of tips fixed to a common substrate layer and an optional support layer, wherein the tips are formed from a high- refractive-index polymer having a refractive index of 1 .65 or greater.

[0031] The high refractive index polymers contemplated can include moldable polymers with a refractive index of greater than 1 .65, greater than 1 .7, greater than 1 .75, greater than 1 .8, greater than 1 .85, greater than 1 .9, greater than 1 .95, greater than 2, or greater than 2.1 . In various cases, the polymer can have a refractive index of 1 .65 to 1 .75, 1 .75 to 2, 1 .6 to 1 .7, 1 .7 to 1 .8, 1 .8 to 1 .9, or 1 .9 to 2. In various other cases, the polymer can have a refractive index of 1 .65 to 1 .7, 1 .7 to 1 .75, 1 .75 to 1 .8, 1 .8 to 1 .85, 1 .85 to 1 .9, 1 .9 to 1 .95, or 1 .95 to 2.

[0032] Contemplated high refractive index polymers include organic polymers or organic- inorganic composite materials comprising an organic polymer matrix and inorganic nanoparticles. In various cases, the inorganic nanoparticles can comprise metal oxide nanoparticles.

[0033] A variety of polymeric materials are contemplated, including polymers of the general classes of silicone polymers, polyester polymers, and epoxy polymers. In various cases, the high refractive index polymer comprises SU-8, poly(p-phenylene ether-sulfone), poly[diphenylmethane bis(4-phenyl)carbonate], polyvinyl phenyl sulfide), poly(styrene sulfide), butylphenol formaldehyde resin, poly(p-xylylene), poly(2-vinylnaphthalene), poly(N- vinyl carbazole), naphthalene-formaldehyde rubber, phenol-formaldehyde resin,

poly(pentabromophenyl methacrylate), or NOA 170. In some cases, the high refractive index polymer comprises SU-8 or NOA 170. In some cases, the high refractive index polymer comprises an organic-inorganic composite comprising a polymer matrix and a metal oxide nanoparticle. In some cases, the metal oxide nanoparticle comprises Ti0 ₂ , ZnO, Zr0 ₂ , or Ce0 ₂ .

[0034] Each tip is coated with a metal layer. In some cases, the metal layer is positioned on each tip to leave an aperture at one end of the tip. In other cases, the metal layer completely coats the tip such that there is no aperture. In various cases, the tips are coated with a metal where the metal comprises gold, silver, aluminum, titanium, or chromium. In some cases, the metal comprises gold. In various cases, the metal layer can have a thickness of 50 nm to 250 nm, 75 nm to 150 nm, or 100 nm.

[0035] In cases where an aperture is present, the aperture in the tip can be formed by any suitable method, including, for example, focused ion beam (FIB) methods or using a lift-off method. In various cases, the tips can be immersed in an etching solution to remove a portion of the metal layer and form the aperture by exposing the material of the tip. The size of the aperture can be controlled during fabrication by appropriate methods known to those skilled in the art. In various cases, the aperture can have a diameter of 1 μηι to 10 μηι, 3 μηι to 10 μηι, 1 μηι to 3 μηι, or 5 μηι to 10 μηι. In various cases, the minimum aperture diameter can be 1 , 1 .5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, or 10 μηι. In various cases, the maximum aperture diameter can be 1 , 1 .5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, or 10 μηι.

[0036] The tip arrays comprise tips which can be designed to have any shape or spacing (pitch) between them, as needed. The shape of each tip can be the same or different from other tips of the array, and preferably the tips have a common shape. Contemplated tip shapes include spheroid, hemispheroid, toroid, polyhedron, cone, cylinder, and pyramid (trigonal or square). The tips have a base portion fixed to the tip substrate layer. The base portion preferably is larger than the tip end portion. The base portion can have an edge length of 1 μηι to 40 μηι, 1 μηι to 20 μηι, 1 μηι to 10 μηι, or 1 .5 μηι to 3 μηι. For example, the minimum edge length can be 1 , 1 .5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20 22, 24, 26, 28, 30, 32, 34, 36, 38, or 40 μηι. For example, the maximum edge length can be 1 , 1 .5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9.5, 10, 1 1 , 12, 13, 14, 15, 16, 17, 18, 19, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, or 40 μηι.

[0037] A tip array can contain 500 or more tips, 1000 or more tips, 2000 or more tips, or up to 10 million tips. In various cases, the tips are arranged in a regular periodic pattern. In various cases, the tips are identically-shaped. In some cases, the tips have a pyramidal shape.

[0038] The tip array can optionally be attached to a support layer, for example, a support layer formed from glass, silicon, quartz, ceramic, polymer, or any combination thereof. The support layer is preferably rigid and has a planar surface upon which to mount the tip array and can act as a rigid support. In some cases, the support layer comprises a glass slide.

[0039] In various cases, the tip array further comprises a common substrate. The common substrate, when present, can comprise any polymer compatible with the high refractive index polymer of the tips. "Compatible with" indicates that neither the polymer of the common substrate nor the high refractive index polymer become unstable or degrade upon contact with each other. In some cases, the common substrate comprises an elastomer. The common substrate can comprise a polymer having linear or branched backbones, and can be crosslinked or non-crosslinked. Cross-linkers refer to multifunctional monomers capable of forming two or more covalent bonds between polymer molecules. Non-limiting examples of cross-linkers include such as trimethylolpropane trimethacrylate (TMPTMA), divinylbenzene, di-epoxies, tri-epoxies, tetra-epoxies, di-vinyl ethers, tri-vinyl ethers, tetra-vinyl ethers, siloxanes {e.g., polydimethylsiloxane or Sylgard® 184), and combinations thereof. In some cases, the common substrate comprises a mixture of polydimethylsiloxane oligomers and crosslinkers {e.g., Sylgard® 184).

[0040] In various cases, the high refractive-index polymer, common substrate layer, and support layer, when present are at least translucent. In various cases, the high refractive- index polymer, common substrate layer, and support layer, when present are transparent.

Fabrication of high refractive index tip arrays

[0041] The tip portion of the tip arrays can be made with a master mold prepared by techniques known in the art {e.g., conventional photolithography and subsequent wet chemical etching), to comprise an array of recesses. The mold can be engineered to contain sufficient recesses to provide as many tips arrayed in any fashion desired. The tips of the tip array can be any number desired, and contemplated numbers of tips include 500 tips to 10 million tips. The number of tips of the tip array can be greater than 500, greater than 1000, greater than 2000, greater than 5000, greater than 10000, greater than 50000, greater than 100000, greater than 500000, greater than 1 million, greater than 2 million, greater than 5 million, or up to 10 million tips.

[0042] The mold recesses can be coated with a metal to form a metal layer. The metal layer can be disposed on the tips by any suitable process, including for example, via an evaporation process to deposit the metal layer. The metal layer can comprise any metal desired, including e.g., gold, silver, aluminum, titanium, or chromium. In various cases, the metal layer comprises gold. Depositing the metal layer can comprise sequential deposition of more than one metal, e.g., deposition of Ti followed by deposition of Au. The metal layer can have any thickness desired. In some cases, the metal layer has a thickness of 50 nm to 250 nm, 75 nm to 150 nm, 90 nm to 1 10 nm, or 100 nm.

[0043] The tip array can be formed by any suitable process via applying a high refractive- index polymer to the mold to fill the recesses. In various cases, applying the polymer comprises coating, e.g., spin-coating, the mold with a high refractive-index polymer. In various cases, the polymer is cured after application to the mold by a suitable method. In some cases, the curing comprises irradiation with UV light. In various cases, a common substrate layer is applied over the mold following curing of the tips. In various cases, applying the common substrate layer comprises coating e.g., spin-coating. In various cases, the common substrate layer comprises polydimethylsiloxane (PDMS). In various cases, a support layer can be applied on top of the common substrate. In various cases, the support layer comprises glass, silicon, quartz, ceramic, polymer, or any combination thereof. In some cases, the support layer comprises glass {e.g., a glass slide). In various cases, the tip array is separated from the mold after application of the support layer.

[0044] An aperture in the metal layer can be formed by any suitable method, before or after application of the high refractive-index polymer. In various cases, an aperture is formed by applying the metal layer to the mold via a tilted rotational evaporation process such that the mold recesses are not coated with the metal layer at the tips and form the apertures when the high refractive-index polymer is subsequently applied. In various cases, an aperture is formed by coating the metal layer over the entirety of the recesses and removing the metal layer from the tips of the tip array to form the apertures after separating the tip array from the mold. In various cases, removing the metal layer from the tips of the tip array comprises applying a protective layer to the tip array such that a portion of the tips of are exposed and removing the metal layer at the exposed tips by etching. Printing with high refractive index tip arrays

[0045] Beam pen lithography can be performed using any suitable platform, for example, a Park AFM platform (XEP, Park Systems Co., Suwon, Korea) equipped with a halogen light source. As another example, a Zeiss microscope can be used with a light source having a wavelength in a range of about 360 nm to about 450 nm. Movement of the tip array when using the Zeiss microscope can be controlled, for example, by the microscope stage.

[0046] In various cases , a high refractive-index polymer tip array as described herein is positioned on or near a photosensitive layer of a surface to be printed, for example, a self- assembled monolayer (SAM) or hydrogel on an Au substrate, followed by exposure {e.g. illumination) of the top surface (e.g., the support layer) of at least one tip, or preferably the tip array, with a radiation source, optionally through a photomask. The radiation is transmitted through the high refractive index polymer tip {i.e., the tip end), thereby printing indicia on the surface.

[0047] The tip array and/or the surface can be moved during patterning to form the desired indicia. For example, in some cases, the tip array is moved while the surface is held stationary. In other cases, the tip array is held stationary while the surface is moved. In still other cases, both the tip array and the substrate are moved.

[0048] The tip array and surface can be leveled with respect to one another . The leveling can be assisted by the transparent, or at least translucent nature of the tip array and tip substrate layer, which allow for detection of a change in reflection of light that is directed from the top of the tip array {i.e., behind the base of the tips and common substrate) through to the surface. The intensity of light reflected from the tips of the tip array increases upon contact with the surface {e.g., the internal surfaces of the tip array reflect light differently upon contact). By observing the change in reflection of light at each tip, the tip array and/or the surface can be adjusted to effect contact of substantially all or all of the tips of the tip array to the substrate surface. Thus, the tip array and common substrate preferably are translucent or transparent to allow for observing the change in light reflection of the tips upon contact with the substrate surface. Likewise, any support layer to which the tip array is mounted is also preferably at least transparent or translucent.

[0049] Radiation can have a wavelength of 200 nm to 800 nm, e.g., 380 nm to 420 nm, for example 365 nm, 400 nm, or 436 nm. In various cases, the radiation comprises UV light e.g. light having a wavelength of 200 to 400 nm. In various cases, the radiation can have a minimum wavelength of 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, or 800 nm. In some cases, the radiation can have a maximum wavelength of 200, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, or 800 nm. [0050] The photosensitive layer of the surface to be printed can be exposed by the radiation transmitted through the polymer tip for any suitable time, for example from 1 second to 5 minutes, or 20 seconds to 120 seconds. In various cases, the minimum exposure time can be 1 , 2, 3, 4, 5 ,6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 1 10, or 120 seconds. In various cases, the maximum exposure time can be 1 , 2, 3, 4, 5 ,6, 7, 8, 9, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 1 10, or 120 seconds.

[0051] The distance between tip array and the surface can form a gap during the irradiating step. In various cases, the gap is 0.5 to 1 .5 μηι, e.g., 0.5, 0.6, 0.7, 0.8, 0.9, 1 .0, 1 .1 , 1 .2, 1 .3, 1 .4, or 1 .5 μηι. In various cases, tip array and the surface do not form a gap {e.g., 0 μηι) during the irradiating step such that the tip array and surface are in contact with each other.

[0052] The surface can be printed using the tip array disclosed herein a plurality of times, wherein the tip array, the surface or both move to allow for different portions of the surface to be irradiated for printing. The time of each contacting step can be the same or different, depending upon the desired pattern. The shape of the indicia or patterns has no practical limitation, and can include dots, lines (e.g., straight or curved, formed from individual dots or continuously), a preselected pattern, or any combination thereof.

[0053] The indicia resulting from the disclosed methods can have a high degree of sameness, and thus can be uniform or substantially uniform in size, and preferably also in shape. The individual indicia feature size (e.g., a dot or line width) is highly uniform, for example within a tolerance of 5%, or 1 %, or 0.5%. The tolerance can be 0.9%, 0.8%, 0.7%, 0.6%, 0.5%, 0.4%, 0.3%, 0.2%, or 0.1 %. Non-uniformity of feature size and/or shape can lead to roughness of indicia that can be undesirable for sub-micron type patterning.

[0054] The feature size can be 10 nm to 1 mm, 10 nm to 500 μηι, 10 nm to 100 μηι, 200 nm to 100 μηι, 200 nm to 50 μηι, 200 nm to 10 μηι, 200 nm to 5 μηι, or 200 nm to 1 μηι. Feature sizes can be less than 5 μηι, less than 4 μηι, less than 3 μηι, less than 2 μηι, less than 1 μηι, less than 900 nm, less than 800 nm, less than 700 nm, less than 600 nm, less than 500 nm, less than 400 nm, less than 300 nm, less than 200 nm, less than 100 nm, or less than 90 nm. In various cases, the feature size is 200 nm to 5 μηι.

[0055] In various cases, during the printing process, a solvent of refractive index less than 1 .35 can be present at (or between) the tip array and photosensitive surface. In various cases, the solvent comprises water, methanol, acetonitrile, or combinations thereof. In various cases, the solvent further comprises a photoinitiator. In some cases, the

photosensitive substrate comprises e.g. an Au surface modified with 1 -dodecanethiol or (1 1 - mercaptoundecyl) tri(ethylene glycol) (EG ₃ ) and present is an aqueous solution containing 1 % (w/v, 34 mM) photoinitiator lithium phenyl(2,4,6-trimethylbenzoyl)phosphinate (LAP)). [0056] The method can further include developing the photosensitive layer, for example by any suitable process known in the art. For example, a printed surface can be contacted with a species, e.g., thiolated oligonucleotides, for further derivatization.

EXAMPLES

Fabrication of SU-8 tip arrays:

[0057] A template-stripping method was employed to fabricate the SU-8 beam pen arrays. Silicon masters with recessed pyramidal microwells (40 μηι edge length, 100 μηι pitch) were prepared according to previous published protocols. A layer of 100 nm Au and a 5-nm Ti layer were sequentially evaporated onto the silicon masters using an electron-beam evaporation system (Kurt J. Lesker Co., USA). SU-8 negative tone photoresist (SU-8 5, MicroChem Inc., USA) was spin-coated onto the metal-coated masters (1000 rpm, 60 s) and soft-baked at 65°C for 2 min and 95°C for 5 min. After exposure to UV light (365 nm, 100 mJ cm-2) and post exposure bake at 65°C for 1 min and 95°C for 2 min, a mixture of polydimethylsiloxane oligomers and crosslinkers (Sylgard® 184, Dow Corning) was poured onto the SU-8 film. Finally, the whole was covered with a glass substrate to form a sandwiched assembly and cured at room temperature for 3 days. The composite tip array was carefully peeled off to obtain the fully metal-coated apertureless SU-8 tips. To make metal-free tips, poly(methyl methacrylate) (PMMA950 A1 1 , MicroChem Inc., USA) was spin- coated onto the metal-coated tip array at 1000-2000 rpm for 60 s followed by baking at 1 15°C for 5 min. The PMMA coating was repeated for one more time to ensure complete coverage. Then the pen array was immersed in an etching solution (Gold Etchant TFA, Transense Company Inc., USA) for 40 s to remove the metal coating on the top parts of pyramids. The spin speed was adjusted to tune the etched portions on the pyramids.

Photochemical Patterning of Self-Assembled Monolayers (SAMs) and Hydroqels:

[0058] Typically, an n-type<100> silicon wafer with 500-nm thermally grown Si02 was evaporated with a layer of 2 nm Ti and 50 nm Au. The Au-coated substrate was immersed in a 1 mM ethanolic solution of 1 1 -mercaptoundecyl tri(ethylene glycol) (EG ₃ , Sigma-Aldrich) at 4°C for at least 24 hrs. Apertureless SU-8 tip arrays were mounted onto a scanning probe platform (XE150, Park Systems) and leveled to the SAM-modified Au substrate optically. A droplet of an aqueous solution of photoinitiator lithium phenyl(2,4,6- trimethylbenzoyl)phosphinate (LAP, Tokyo Chemical Industry Co., Ltd.) was injected into the area between the tip arrays and the substrate. An optical fiber coupled with a mercury lamp (X-Cite 120 Q, Excelitas Technologies Corp.) was used to illuminate the tip arrays from the backside (365 nm, Pmax=0.25 Wcm-2), and patterns were generated with the simultaneous movement of piezoelectric actuators. Hydrogels was made by photopolymerization of poly(ethylene glycol) diacrylate (PEGDA, average Mn 700, Sigma-Aldrich) with 2,2- dimethoxy-2-phenylacetophenone (DMPA, 1 wt%) as the photoinitiator. The as-made hydrogel was placed in a petri dish and covered with an aqueous solution of 1 mM

Rhodamine-labeled PEG thiol (Mn 5000, Nanocs Inc.) and 34 mM LAP. Then patterning with SU-8 tip array on the hydrogel surface was conducted through 365 nm UV illumination (~ 10 mW cm-2, 1 min). The patterned hydrogel was thoroughly rinsed with Dl water and characterized with fluorescence microscopy.

Generation of Au Patterns and DNA-Nanoparticle Arrays:

[0059] After photopatterning, the Au substrate was placed in an etching solution consisting of 20 mM thiourea, 30 mM iron nitrate, 20 mM hydrochloric acid, and 2 mM octanol in water for 3 min to yield hole features of Au. Alternatively, the Au substrate was immersed in a single-strand DNA solution (1 μΜ, 1 M NaCI) for 1 h, followed by the hybridization with the complementary linkers and the attachment of oligonucleotide-modified gold nanoparticles. Patterns were characterized using optical microscopy (Axiovert-Zeiss), atomic force microscopy (Dimension Icon, Bruker), and scanning electron microscopy (SU8030, Hitachi).

Simulations:

[0060] FDTD simulations were performed using a commercial package (Lumerical FDTD solutions v.8.1 1 .337). The refractive indices of environment and SU-8 pyramid were assumed to be 1 .34 and 1 .65, respectively, around the wavelength of the light source. The pyramid height to base edge ratio was 0.707 and the pyramid base edge size in the simulation was reduced to 3 μηι due to computational limitation. The total field scattered field (TFSF) plane wave source was used to avoid the light interaction with the simulation boundary. The light polarization was parallel to the base edge direction of the pyramid. The wavelength of the injected light was 350-450 nm (a pulse in the time domain). The Perfectly Matched Layers (PML) boundary condition was used to absorb the electromagnetic fields at the simulation boundary. Electric fields were recorded in five different 2D monitors in the time domain following the light pulse injection, and they were Fourier-transformed into the frequency domain to generate the local intensity profile. Three horizontal 2D monitors were located at the top, middle and bottom planes of the photoresist, and two vertical monitors were parallel to the pyramid base edges. The spectral profile of the light source (LED) was addressed by averaging the intensity. The broadband results are convoluted with a narrow weighting function (Amax~400nm) to analyze the response around wavelength of 400 nm. All the simulation images are obtained at 400 nm. REFERENCES

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