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. Author manuscript; available in PMC: 2024 Jun 27.
Published in final edited form as: Langmuir. 2023 Dec 28;40(1):439–449. doi: 10.1021/acs.langmuir.3c02700

Transfer Printing of Ordered Plasmonic Nanoparticles at Hard and Soft Interfaces with Increased Fidelity and Biocompatibility Supports a Surface Lattice Resonance

Keith R Berry Jr 1,⊥, Donald Keith Roper 2,⊥, Michelle A Dopp 3, John Moore II 4
PMCID: PMC11209850  NIHMSID: NIHMS1999634  PMID: 38154131

Abstract

Transfer printing, the relocation of structures assembled on one surface to a different substrate by adjusting adhesive forces at the surface–substrate interface, is widely used to print electronic circuits on biological substrates like human skin and plant leaves. The fidelity of original structures must be preserved to maintain the functionality of transfer-printed circuits. This work developed new biocompatible methods to transfer a nanoscale square lattice of plasmon resonant nanoparticles from a lithographed surface onto leaf and glass substrates. The fidelity of the ordered nanoparticles was preserved across a large area in order to yield, for the first time, an optical surface lattice resonance on glass substrates. To effect the transfer, interfacial adhesion was adjusted by using laser induction of plasmons or unmounted adhesive. Optical and confocal laser scanning microscopy showed that submicron spacing of the square lattice was preserved in ≥90% of transfer-printed areas up to 4 mm2. Up to 90% of ordered nanoparticles were transferred, yielding a surface lattice resonance measured by transmission UV–vis spectroscopy.

Graphical Abstract

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1. INTRODUCTION

Electronic and/or optical components can be transfer printed onto a flexible, working substrate by adjusting mechanical (kinetic, shear, structural) or physicochemical (thermal, optical, chemical) forces to change interfacial adhesion.1-3 Ordered micro- and nanostructures have been transfer printed onto skin, eyes, or leaves4,5 to produce flexible and transient electronic circuits with, e.g., transistors, capacitors, memory, sensors, energy harvesters, and light-emitting diodes.6-8 Ordered nanostructures9 are typically fabricated on rigid surfaces by electron beam lithography,10,11 nanotribological printing,12 dip-pen nanolithography,13 laser writing,14 or similar top-down15-17 approaches. However, such approaches use extreme conditions (i.e., high temperature, high vacuum, strong solvents) incompatible with biological tissues. Transfer printing of nanofabricated structures onto skin or plant substrates could avoid conductive heat, high-powered lasers, or adhesive backing typical of alternatives like microcontact printing, direct laser writing, or tape nanolithography.5,14,18-26 To date, however, biocompatible transfer printing of nanostructures at high fidelity over large areas sufficient to maintain optical functionality that depends on regular submicrometer features is not reported.

Ordered nanoparticles (NPs) have the potential to improve sensing in flexible electronics.5-8,18,24-28 Two-dimensional (2D) lattices of NPs can couple resonant plasmons induced on the NP to incident diffracted light and result in an optical surface lattice (Fano) resonance.29,30 The NP composition, NP geometry, lattice spacing, and incident light wavelength can be tuned so that the surface lattice resonance has better signal-to-noise and higher sensitivity to refractive index changes than conventional surface plasmon resonance or field effect sensors.31-36 Transfer printing of ordered NPs to support a surface lattice resonance, however, has not been reported. Tape nanolithography was used to pattern NPs on a biological (leaf) surface.5,18,24,26 But residual tape backing at the interface could interfere with leaf function and possible sensor response.

This work developed two new biocompatible methods to transfer a nanoscale square lattice of plasmon resonant NPs from a lithographed surface onto soft (leaf cuticle) and hard (soda lime glass) substrates with sufficient fidelity and area to yield, for the first time, an optical surface lattice resonance on glass. To adjust interfacial adhesion and effect transfer, the first method combined laser induction of plasmons with a biocompatible solvent; the second method used a biocompatible unmounted adhesive. The degree of preserved order, area of high-fidelity transfer, and effect on leaf viability post-transfer were analyzed microscopically and optically. The laser- and adhesive-based methods transferred up to 90% of ordered NPs across 100 μm2 to 4 mm2 areas. UV–vis spectroscopy measured an optical surface lattice resonance from nanoscale square lattices of NPs transferred onto glass, the first such feature produced by transfer printing an ordered NP lattice. These results show that transfer printing ordered NPs can create an optical feature to improve sensing in flexible and transient electronic circuits.

2. EXPERIMENTAL SECTION

2.1. Fabrication of the AuNP Lattice.

Gold (Au) NPs were deposited in a square lattice of cylindrical cavities lithographed onto a poly(dimethylsiloxane) (PDMS) surface. Evaporative self-assembly was used to fill up to 67% of cavities with AuNPs across areas >100 μm2.20,37 Briefly, PDMS was cured onto a silicon master with an area of 8 mm × 8.3 mm that contained 150 nm × 195 nm (height × diameter) cylindrical pillars in a square lattice at a pitch of 600 nm (Lightsmyth Technologies, S2D-24B2-0808-150-P). Removing the master from the cured PDMS yielded an 8 mm × 8.3 mm PDMS stamp containing a square, nanoscale lattice of cavities of ~150 nm × 195 nm (depth × diameter). Spherical AuNPs of a 100 nm diameter (Nanopartz, AC11-100-CIT-DIH-100-1) were diluted to 3 mg/mL in aqueous 1% Triton X-100 (Sigma-Aldrich, X-100-5 ML). The PDMS stamp was bisected and placed on a motorized linear stage. A 2.5 μL drop of 1% Triton X-100 was added to the edge of the stamp. A glass superstrate was lowered onto the drop to spread it across the bottom third of the stamp. The stage was moved parallel to the stamp surface at 8 μm/s to deposit the surfactant on the stamp as liquid from the drop evaporated. Next, a 2.5 μL drop of the AuNPs was added and spread by lowering the superstrate and then evaporated by moving the stage at 1.2 μm/s. This deposited a single AuNP in up to 67% of cavities across areas up to ≥2 mm × 2 mm. These ordered AuNPs were transfer printed onto leaf and glass using the below methods.

2.2. Transfer Printing via Laser Induction.

The PDMS stamp containing AuNPs deposited in a square lattice of cavities was first coated with 20% Triton X-100 using a motorized linear stage at 8 μm/s. A 3 μL drop of ethanol (95%, V1101, Decon Laboratories, Inc.) was deposited on the substrate (soda lime glass coverslip (VWR, 48366-089) or leaf cuticle). The PDMS stamp was inverted onto the ethanol-coated substrate. Light weights (<10 g) were placed atop the PDMS stamp to promote surface–substrate contact. The PDMS–substrate interface was then irradiated for 10 min with a 532 nm laser at 50 mW through an optical chopper operating at 6000 Hz (Thorlabs, MC2000B). Next, the PDMS was removed, leaving ordered AuNPs atop the substrate. Scheme 1 illustrates this process and Figure 1 shows its results (see Section 3.1).

Scheme 1. Resonant Laser Induction to Transfer Print AuNPs Ordered atop a Structured PDMS Surface onto (i) Hard (Glass, Upper) and (ii) Soft (Leaf Cuticle, Lower) Substratesa.

Scheme 1.

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aAdding cyanoacrylate preserved AuNP order but discolored (browned) the leaf cuticle.

Figure 1.

Figure 1.

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Dark-field images of AuNPs transfer printed by laser induction onto (a) glass and (b–f) leaf cuticle. Scale bar ~3 μm, except where noted. Ordered AuNPs in small patches atop tabular cells (b) and along veins (e). Disordered AuNPs atop an open stoma pore (c). Clustered AuNPs atop the leaf cuticle (d) (scale bar ~30 μm). The leaf cuticle discolored after cyanoacrylate was added (f) (scale bar ~1 mm).

2.3. Transfer Printing via Shellac.

Shellac, C30H50O11 (Zinsser Bulls Eye Shellac Traditional Finish and Sealer aerosol spray), was sprayed on a clean soda lime glass microscope slide (VWR, 16004-430) in two 1 s passes at a distance of 250 mm. A 4 mm × 8.3 mm PDMS stamp containing a self-assembled AuNP lattice was immediately inverted onto the shellaced slide and cured for 1.5 min at ambient conditions. The stamp was then removed from the shellaced slide and cured face up for 1.5 min at ambient conditions. Next, the stamp was inverted onto either a glass coverslip or leaf cuticle substrate and gently pressed with a finger for 1 min. Finally, the stamp was slowly peeled from the glass or leaf, starting at one corner of the stamp, leaving behind a thin layer of shellac to which the ordered array of AuNPs adhered. Scheme 2 shows this process, and Figures 2 and 3 show its results (see Section 3.4).

Scheme 2. Transfer Printing via Shellac of AuNPs Inside a Square Lattice of Cavities in PDMS onto (I) Hard Glass Coverslips (above) and (ii) Soft Leaf Cuticle Surfaces (below).

Scheme 2.

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Figure 2.

Figure 2.

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Dark-field optical microscopy images of transfer printing via shellac: (a) ordered shellac transfer printed on glass. (b) Ordered AuNPs in shellac transfer printed onto the glass. (c) Ordered AuNPs in shellac transfer printed onto the cuticle. (Scalebars: ~3 μm).

Figure 3.

Figure 3.

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LSCM 2D and 3D images of (a) a silicon master stamp containing ordered pillars used to produce PDMS stamps with ordered cavities, (b) without AuNPs, and (c) with evaporatively deposited AuNPs filling the cavities. Resin transfer printing from unfilled cavities in (b), produced ordered Shellac pillars without AuNP as in (d). Resin transfer printing from filled cavities as in (c) produced ordered AuNPs transferred onto (e) silica and (f) cuticle surfaces. The pillar (g) diameter, (h) interpillar lattice spacing, and (i) height were all measured for (a-f) and are included in Table 2.

2.4. Optical and Spectroscopic Characterization.

The appearance, NP density and ordering, and optical activity of transfer-printed substrates were characterized by using optical microscopy, transmission ultraviolet–visible (TUV–vis) spectroscopy, and confocal laser scanning microscopy.

2.4.1. Optical Microscopy.

Reflection dark-field images were obtained using an optical microscope (Nikon Instruments, Eclipse LV100). Ordered nanostructures were imaged at 100×. Leaf surfaces were imaged at 5×.

2.4.2. UV–Vis Spectroscopy.

Transmission spectra of ordered nanostructures were measured by using a light microscope (Nikon Instruments, Eclipse LV100) integrated with a UV–vis spectrometer (Andor Technology, Shamrock 303). This enabled transmission spectra of each AuNP-decorated PDMS sample to be obtained at the same location as the corresponding dark-field image.

2.4.3. Laser Scanning Confocal Microscopy (LSCM).

Images of stamp and PDMS surfaces and glass and leaf substrates were collected with a Keyence VK-X1000, as shown in Figure 3a-f, and analyzed using stock Keyence software to measure feature sizes, as shown in Figure 3g-i. Measuring the height (depth) of cavities lithographed into PDMS by the silicon stamp by LSCM proved difficult (Figure 3). Table 2, rows (b) and (c), show values of 20 and 30 nm for cavity height without or with AuNP, respectively. This could be due to the optical transparency of PDMS, which reduced the accuracy of 3D measures of cavity depth and/or collapse of cavities during the energetic LSCM measurement process.

Table 2.

Dimensions of Pillars/Cavities (N = 842) of Original Surfaces and of Transfer-Printed Substrates from Images as Shown in Figure 3 Measured by LSCM. See Section S3 for Details

surface row example figure surface feature diameter (nm) height (nm) spacing (nm)
silicon stamp (a) 3a cylindrical pillar 201.3 ± 2.5 153.7 ± 1.9 600.0 ± 0.8
PDMS surface (b) 3b cylindrical cavity 201.7 ± 5.4 20.3 ± 0.5 602.3 ± 4.9
PDMS + AuNP (c) 3c filled cavity 201.7 ± 5.4 29.7 ± 5.3 599.0 ± 6.7
shellac on glass (d) 3d cylindrical pillar 195.3 ± 11.6 150.0 ± 2.0 604.0 ± 12.5
shellac + AuNP on glass (e) 3e cylindrical pillar 198.7 ± 5.0 150.0 ± 2.0 602.7 ± 2.1
shellac + AuNP on leaf (f) 3f cylindrical pillar 197.7 ± 9.3 150.3 ± 3.1 605.7 ± 3.2
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2.4.4. ImageJ Analysis of AuNP Lattices.

Color tones (dark, gray, bright) of LSCM images were analyzed in ImageJ to determine the AuNP status (single, missing, multiple) at lattice grid points, as detailed in Section S.3. Briefly, 67% of PDMS cavities analyzed near edges of PDMS stamp contained a deposited AuNP. At corresponding edges of transfer-printed substrates, 60% of shellac pillars contained AuNPs. Thus, the percent of AuNPs transfer-printed via shellac at the edges of the deposition area was 90%. Comparing images like Figure 3c,e showed that >90% of AuNPs were transferred in the inner areas of the PDMS stamp. Transfer-printed vacancy rates greater than 10% were typically due to the PDMS having <90% occupancy, as observed in Figure 3c.

3. RESULTS AND DISCUSSION

3.1. Transfer Printing via Laser Induction.

A PDMS surface containing AuNPs deposited in a square lattice of cavities was inverted onto a glass or leaf substrate. Irradiating the PDMS–substrate interface by a low-power 532-nanometer (nm) laser induced a localized surface plasmon (LSP) on the AuNPs. Scheme 1 shows steps in transfer printing via laser induction. Resonant LSP energy was dissipated via phonon heating of the biocompatible solvent (ethanol) deposited at the interface.38-42 This noncontact optothermal force, localized to the AuNP and adjacent solvent, adjusted the interfacial adhesion to transfer print the submicron AuNP lattice onto the substrate.

Low-power laser induction increased biocompatibility compared to previously reported high-power pulsed laser14 or conventional20 heating. Dissipation of LSP energy confined heating to the AuNPs and solvent at the interface, minimizing collateral heating of the adjacent PDMS surface and substrate (glass or leaf cuticle).41,43,44 Ethanol was preferred as the chemical solvent20 to wet PDMS/glass and PDMS/leaf cuticle interfaces due to its biocompatibility. Solvating with deionized distilled (DI) water, 50:50 DI water/ethanol, 80:20 DI water/ethanol, or 90:10 DI water/ethanol did not transfer the AuNP onto leaf cuticle substrates. Brief ethanol exposure to cellular tissues protected by an organic coating (e.g., waxy leaf cuticle coat, described in Section 3.2) proved innocuous. Negligible residual ethanol remained after rapid evaporation during LSP dissipation at ambient temperature and pressure. Laser induction reduced the time required for heating and reduced the solvent exposure due to rapid evaporation. This preserved the coloration and stomatal opening on the leaf. Stomatal closing is a leaf response to external stress. In contrast, Bagheri et al. used a femtosecond pulsed laser to selectively etch a Au coat from a calcium difluoride (CaF2) substrate at high vacuum.14 Cerf et al. used ethanol and water solvents to transfer AuNPs from a patterned PDMS stamp to APTES-coated glass in an oven at 100 °C.20 Neither a femtosecond laser at high vacuum nor 100 °C oven heating would preserve leaf viability.

Transfer printing by laser induction onto the glass substrate retained nodal fidelity (fraction of ordered AuNPs transferred) and geometric fidelity (square lattice arrangement of AuNPs transferred). Dark-field microscope images, as in Figure 1a, indicated that 90% of the ordered AuNPs deposited in cavities of the PDMS surface were transferred to the glass substrate. Figure 1a is a reflected dark-field image of a ~25 μm × 25 μm subsection of a larger ≥2 mm × 2 mm transfer-printed area. In dark-field microscopy, scattered light from an individual AuNP appeared as a white dot. The lattice ordering of the white dots reflected the square lattice arrangement of the silicon-lithographed PDMS. Such an ordering produced visible iridescence (Figure S1). The absence of an AuNP resulted in a dark vacancy. LSCM showed that vacancies in Figure 1a arose from incomplete filling of PDMS cavities via evaporative deposition rather than from either incomplete transfer printing or incomplete nanolithography of PDMS by the silicon stamp. Figures 3a, 3b, and 3d show no missing pillars on the silicon master, no missing PDMS stamp cavities, and no missing shellac pillars on the glass, respectively. Section 2.4 ImageJ analysis of AuNP lattices details analysis of AuNP vacancies in images. Transfer printing by laser induction onto a leaf cuticle substrate maintained leaf surface structures (stoma, veins, tabular cells) and coloration, but lattice fidelity was disrupted outside small areas. Dark-field microscopy showed that transferred AuNPs appeared as patchworks of either small areas (≤15 μm × 15 μm) with ordered AuNPs, as in Figure 1b,e, larger areas with disordered or clustered AuNPs, as in Figure 1d,e. Figure 1c shows a small area of semiordered AuNPs in and around a stoma on the cuticle surface. Across the leaf cuticle, patches similar to those in Figure 1b,c varied in size up to 15 μm × 15 μm. This appeared due to the uneven surface topography of the leaf cuticle, which resulted in variable contact between the cuticle surface and the PDMS stamp. The addition of small weights (<10 g) atop the PDMS stamp improved the number of AuNPs transferred but reduced the order, as shown in Figure 1d. ImageJ analysis of AuNPs on surfaces before and after transfer, discussed in Section 2.4, indicated that up to ≥85% of AuNPs assembled on a PDMS stamp were transfer printed to the leaf cuticle but without preserving the order of the square lattice.

3.2. Biochemistry at the PDMS/AuNP–Leaf Interface that Enabled Transfer.

The leaf substrate for transfer printing was the outermost, waxy, protective layer on the epidermis of an Apocynum cannabinum leaf known as the cuticle.45 Figures 1f and S4 show that the abaxial leaf surface—the leaf underside that is opposite the stem—is smooth with a prominent network of white veins. The cuticle is more dense (458 μg/cm2) on the abaxial surface than on the upward-facing (adaxial) surface (305 μg/cm2). Moreover, stomata, as shown in Figures 1c, S2b, and S3b, occur on the abaxial surface at a density of 320 stomata/mm2.46,47 Epicuticular wax comprises 85 and 56 μg/cm2 of cuticle on the adaxial and abaxial surfaces.

The interfacial tension between the hydrophobic cuticle substrate and the PDMS surface embedded with AuNPs was lowered by coating PDMS with the surfactant Triton X-100 in all data shown before depositing it on the cuticle.48,49 Hydrophobic and poly(ethylene glycol) arms of Triton X-100 (see Table 1) are capable of bridging the hydrophobic cuticle and hydrophilic AuNPs and capping agents through van der Waals and hydrogen bonding, respectively.50,51 Triton X-100 is reported to comodify citrate-stabilized AuNPs, improving dispersibility and solubility.52,53 This facilitated transfer printing of AuNPs from PDMS onto the cuticle, as seen in Figures 1b-e and 3f. Figure S6 illustrates Triton X-100 at the interface bridging the hydrophilic AuNP citrate capping agent and hydrophobic cuticle components. No AuNP was observed to transfer by laser induction from PDMS to the leaf cuticle without first coating the AuNP–PDMS surface with 20% Triton X-100 before contacting it to the cuticle.

Table 1.

Primary Constituents of the Leaf Substrate and (Bio)Chemicals used to Promote Transfer Printing to Surfaces

Constituent Formula/Structure
Leaf surface: cutin and epicuticular wax constituents
Hydroxy fatty acid HOCH2(CH2)14COOH
Hydroxy fatty acid CH3(CH2)8HOCH(CH2)5COOH
Straight-chain alkane CH3(CH2)nCH3, n=27,29
Fatty acid ester CH3(CH2)22COO(CH2)25CH3
Long-chain fatty acid CH3(CH2)22COOH
Long-chain alcohol CH3(CH2)24CH2OH
Surfactant
Triton X-100* CH3C(CH3)(CH3)CH2C(CH3)(CH3)C6H4(OCH2CH2)nOH
AuNP capping agent
Citrate C3H5O(COO)33−
Adhesive
Ethyl cyanoacrylate CH2C(CN)COOCH2CH3
Polymethacrylate −[CH2C(CH3)(COOR)]n−
Hydroquinone HOC6H4OH
Shellac
Aleuritic acid graphic file with name nihms-1999634-t0008.jpg
Jalaric acid-A graphic file with name nihms-1999634-t0009.jpg
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*

n = 9–10.

The cuticle biochemistry potentiated van der Waals bonding to hydrophobic Trixon X-100 arms, hydrogen bonding to adhesives, and cyanoacrylate polymerization.54,55 Table 1 lists the primary constituents of cutin polyester and epicuticular wax comprising the leaf cuticle.56-58 These are long-chain alcohols, alkanes, fatty acids, fatty acid esters, and hydroxy fatty acids. Highly polar nitrile and ester groups of ethyl cyanoacrylate, which comprises 60–95% of fast-acting adhesives, are strong hydrogen bond acceptors.59 Hydroxides in moisture at the cuticle-PDMS interface catalyze rapid nucleophilic chain polymerization of cyanoacrylate, accelerated by cuticle fatty groups.60 Methacrylate thickening agents in the cyanoacrylate adhesive are water-soluble and miscible in organic solvents, furthering PDMS–cuticle interaction.61 Weakly acidic para hydroxyl groups on the polymerization inhibitor hydroquinone that comprise <0.5% of cyanoacrylate adhesive also bridge hydrogen bonding.62

A drop (~5 mg) of the cyanoacrylate-based adhesive added to the 20% Triton X-100 used to coat the PDMS decreased leaf cuticle viability. Water lost in cyanoacrylate polymerization resulted in the browning of the leaf, as evident in Figure 1f. Rapid closing of the stomata, as shown in Figure 1c in the area where adhesive-doped Triton was deposited, was observed during microscopic characterization immediately after transfer printing. Because cyanoacrylate damaged the leaf, it was not used further.

The deterioration of the leaf due to cyanoacrylate motivated the search for a more biocompatible unmounted adhesive to promote a high-fidelity transfer. Consideration of several alternatives led to the selection of shellac. Shellac is a highly colored resin secreted by a female lac bug on trees in the forests of India and Thailand. It is bleached to optical transparency before being dissolved in a mixture of alcohol, organic acids, and ketones for use. Shellac is used broadly to preserve plant63 and mammalian64 tissues. It is approved by the U.S. Food and Drug Administration for food- and drug-related applications. Its film-forming, water-resistant, and barrier properties minimize structural distortion or discoloration of shellac-coated tissue. Shellac is an amphiphilic oligomer composed of aliphatic long-chain fatty acids and sesquiterpenes.65 It consists primarily of aleuritic acid (C16H32O5) and jalaric acid-A (C15H20O5) building blocks in a nearly 1:1 ratio connected by lactide and ester linkages.66,67 Table 1 shows the structures of these acids. The resinous consistency of shellac is due to the association of aleuritic acid and jalaric acid-A (a cyclic terpene acid) through van der Waals and hydrogen bonding interactions.68 Multiple proton-accepting alcohols and proton-donating carboxylic acids on shellac moieties are capable of interacting with fatty acids and alcohols, respectively, on the leaf cuticle.

3.3. Transfer Printing via Shellac.

A PDMS surface containing AuNPs deposited into a square lattice of cylindrical cavities was inverted onto a shellac-coated slide to adjust interfacial adhesion and then cured at ambient conditions. Scheme 2 illustrates steps in transfer printing via shellac. The shellac-coated PDMS surface was subsequently inverted onto either the glass coverslip or the leaf cuticle substrate. Slowly peeling the PDMS surface away from the substrate yielded an ordered array of shellac pillars containing AuNPs on a glass substrate (see Figure 2b) or a leaf cuticle substrate (see Figure 2c). By comparison, Figure 2a shows shellac pillars free of AuNPs transferred to a glass substrate. Transfer printing via shellac resulted in ≥90% transfer inside the edges of the transfer area, as detailed in Section 2.4. Vacancies in Figure 2b arose primarily from the incomplete filling of PDMS cavities via evaporative deposition, as detailed in Section 3.1

Transfer printing via shellac proved biocompatible with the leaf, with no browning or deterioration observed following transfer (see Figures 2c and S3). Shellac is frequently used to preserve the color and character of plant and animal tissue. The absence of moisture-catalyzed nucleophilic chain polymerization that was observed with cyanoacrylate helped preserve leaf color. Abundant alkanoic and proton-donating/accepting groups on aleuritic acid and jalaric acid-A in shellac-potentiated hydrogen and van der Waals bonding with the long-chain alcohols, alkanes, fatty acids, fatty acid esters, and hydroxy fatty acids of the cutin polyester and epicuticular wax that comprised the leaf cuticle. Figure S7 illustrates a shellac bridging hydrophilic AuNP citrate capping agent to hydrophilic and hydrophobic cuticle components. The biocompatibility of shellac has led to its use in intravascular devices,69 cell scaffolds,70 and controlled-release microcapsules71 where shellac maintained the viability of cells exposed to acid. Other properties of shellac that enabled transfer printing were its solubility, conformality, aggregation, compaction, adherence, and mechanical rigidity upon drying. Shellac dissolved in low-molecular-weight alcohols, and acetone conformed readily to the nanometer-scale dimensions of the cavities stamped into the PDMS surface. A short ambient cure evaporated acetone and volatile alcohols. Evaporation allowed the aggregation of aleuritic acid and cyclic terpene acids into a continuous shellac phase with mechanical rigidity. Mechanical compaction of conformed pillars due to evaporation and aggregation reduced interaction with the PDMS template. Fatty acids and sesquiterpenes at the nonconformed surface offered van der Waals and hydrogen bonding interactions to adhere the conformed shellac coating to a glass or cuticle substrate. The sequence of solubilization, conformation, aggregation, compaction, and adherence enabled high-fidelity transfer printing via shellac, as illustrated in Figure 2a.

3.4. Fidelity of Transfer Printing via Shellac.

LSCM was used to quantify the fidelity of transfer printing. Figure 3 shows LSCM images of (a) the silicon master stamp with pillars of a nominal 195 nm diameter and 150 nm height squarely spaced 600 nm apart; (b) a PDMS stamp with a square lattice of vacant cavities corresponding to the silica master; (c) a PDMS stamp with cavities filled by evaporatively deposited AuNPs; (d) shellac pillars without AuNPs transfer printed onto glass; (e) shellac pillars containing AuNPs transfer printed onto glass; (f) shellac pillars containing AuNPs transfer; and measurements of the (g) pillar diameter; (h) pillar spacing; and (i) pillar height that were performed for (a)–(f). Table 2 shows data obtained from images such as those shown in Figure 3.

Shellac that was transfer printed without AuNPs largely retained its conformal structure. The LSCM image in Figure 3a shows the silicon master stamp with pillars of a nominal diameter of 195 nm, height of 150 nm, and spacing of 600 nm. Corresponding measures in Table 2, row (a), slightly exceeded the nominal values due to residual PDMS cured on the surface of the pillars. Figures 2a and 3d show dark-field microscopy and LSCM images, respectively, of shellac (containing no AuNPs) transfer printed from a lattice of empty PDMS cavities, as in Figure 3b, to a glass substrate. LSCM measures in Table 2, row (c), show that the resulting ordered pillars had a diameter, height, and spacing nearly identical to nominal values.

Shellac containing AuNPs that were transfer printed yielded features with dimensions near those of the original lithographed surface. Figures 2b and 3e show dark-field microscopy and LSCM images of ordered AuNPs transfer printed via shellac onto the glass substrate. Figures 2c and 3f show similar images for ordered AuNPs transferred onto leaf cuticle substrates. Table 2, rows (e) and (f), show that the diameter, height, and spacing of transfer-printed shellac pillars containing 100 nm AuNPs were slightly smaller than those of the PDMS cavities in Table 2, rows (b) and (c), on both the glass and leaf. The exception was cavity height (depth), as shellac-free cavities appeared to collapse during LSCM measurements. Shellac pillar dimensions were slightly smaller than cavities in PDMS. This was attributed to mechanical compaction of the shellac pillar within the PDMS cavity as evaporation promoted aggregation of aleuritic acid and cyclic terpene acids.72

Transfer printing via shellac maintained lattice spacing and AuNP occupancy on glass (Figure 2b) and leaf cuticle (Figure 2c) substrates despite uneven surface topography of the leaf. Comparing the number of AuNPs transferred onto the substrate vs the number of AuNPs deposited in PDMS cavities showed ≥90% transfer across areas of ≥2 mm × 2 mm, as detailed in Section 2.4 ImageJ analysis of AuNP lattices. In contrast, transfer printing by laser induction (Figure 1b-e) infrequently maintained lattice spacing on the leaf in small areas. Shellac compensated for irregularities at the cuticle surface, maintaining the geometric order of AuNPs while adhering conformally to rounded convex tabular cells, stomata, and veins.

3.5. Optical Activity of Transfer-Printed Ordered AuNPs.

A sharp surface lattice (Fano) resonance (SLR) is observed from ordered lattices of plasmonic NP at certain conditions due to the coupling of localized surface plasmon resonance (LSPR) oscillations with collective narrow-band optical diffraction at a lattice-constant wavelength.73,74 The SLR has more signal-to-noise and higher sensitivity to changes in local refractive than LSPR or field effect sensors, thus offering improved sensing for flexible electronics on biological surfaces.76 Preserving the lattice grating constant and occupancy of AuNP at lattice nodes are key to maintaining SLR signal-to-noise and sensitivity.75 Previous methods reported to print or transfer ordered NP patterns yielded LSPR signals but not SLR.5,18,24,26

Transfer printing via laser induction and shellac yielded the first such SLRs enabled by transfer printing, in addition to measurable LSPRs. Figure 4 shows transmission spectra for visual (vis) to near-infrared (NIR) wavelengths from ordered shellac pillars on glass (blue line); AuNPs deposited in ordered PDMS cavities (green line); ordered AuNPs in shellac pillars on glass (orange line), and laser-induced ordered AuNPs on glass (gray line). AuNPs of a 100 nm diameter in a 600 nm square lattice support an SLR at wavelengths >680 nm and an LSPR at wavelengths <680 nm at conditions of the substrates. Dark-field 100x microscope images in Figure 4 illustrate square lattices from which spectra were obtained, as indicated by their border color. Table 3 summarizes the data for images in Figure 4 and corresponding spectral features. These data are percent AuNP-occupying nodes and SLR and LSPR wavelengths and extinction intensity. Each image corresponds to an 18 μm × 13.8 μm (e.g., 250 μm2) sample containing 690 pillars or cavities taken at the center of a 2 mm2 spot from which spectra shown in Figure 4 were obtained. The total area transfer printed for each sample was 4 mm2, as illustrated in Figure S1, except for shellac on glass, which was 8 mm2.

Figure 4.

Figure 4.

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Transmission vis-NIR spectra of ordered shellac pillars on glass (blue line), 100 nm AuNPs deposited in ordered PDMS cavities (green line), ordered 100 nm AuNPs in shellac pillars on glass (orange line), and laser-induced ordered 100 nm AuNPs on glass (gray line). Dark-field 100× microscopy images at the center of the spot at which each spectrum (see colored borders) was measured are on the right (scale bar ~4 μm).

Table 3.

Data for Figure 4: Image Description and Corresponding Spectral Features

sample shellac pillars on glass AuNPs in PDMS before transfer AuNPs in shellac pillars on glass AuNPs on glass
transfer print method shellac - shellac laser
Color of data line in Figure 4 blue green orange gray
AuNPs occupying nodes (%) 0 72 ± 4 50 ± 6 62 ± 4
peak wavelength: λLSPR (nm) - 553 ± 5 576 ± 0.4 557 ± 17
extinction: ELSPR (A.U.) - 0.075 ± 0.002 0.049 ± 0.003 0.069 ± 0.006
peak wavelength: λSLR (nm) - 773 ± 26 817 ± 9 850 ± 10
extinction: ESLR (A.U.) - 0.062 ± 0.014 0.035 ± 0.009 0.116 ± 0.016
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AuNPs deposited in ordered PDMS cavities (green line and box) before transfer printing had the most AuNP-occupying nodes in the square lattice (Table 3, column 2). AuNPs in PDMS cavities exhibited an SLR peak at 773 nm and an LSPR peak at 553 nm. This LSPR peak had a higher extinction than LSPRs from AuNPs on glass (gray) or in shellac (orange) due to more occupied nodes. This LSPR peak had higher energy (lower wavelength, 553 nm) than others since PDMS (refractive index (RI) = 1.42) damps plasmons less than glass (RI = 1.46) or shellac (RI = 1.52–1.53). The SLR peak height was similar to the LSPR because of vacant nodes as well as the variety of media surrounding the particle (RIPDMS = 1.42, RIAir = 1.0, RITriton = 1.49, RIcitrate = 1.58) and its complex geometry.

AuNP transferred in shellac pillars onto glass (orange line and box) occupied fewer nodes than PDMS as only 90% of AuNPs transferred (Table 3 column 3). So SLR and LSPR peak heights were lower than in PDMS. The SLR and LSPR red-shifted to 817 and 576 nm, respectively, due to higher RIs of glass (1.46) and shellac (1.52–1.53) compared to PDMS (1.42). The SLR red shift was larger as it is more sensitive to external RI.

AuNPs transfer printed via laser induction onto glass (gray line and box) also occupied fewer nodes than PDMS, yielding a lower LSPR (see Table 3, column 4). The SLR peak was higher, however, as AuNPs were surrounded mostly by air. The SLR and LSPR red-shifted to 850 and 557 nm, respectively, compared to PDMS, as the higher glass RI reduced the speed and wavelength of light so that the SLR diffractively coupled at lower vacuum energies (correspond to larger lattice constants). Similarly, Matricardi et al. showed that larger lattice constants of AuNP clusters red-shifted the SLR features in the infrared.77

Ordered shellac pillars (blue line and box) that were not occupied by AuNPs (see Table 3, column 1) caused optical diffraction and scattered light at progressively lower wavelengths (higher energies) but did not produce LSPR. So, neither an SLR nor LSPR appeared for empty pillars. Leaves did not transmit sufficient light to measure LSR or LSPR via transmission UV/vis.

3.6. Merits of Transfer Printing.

Alternatives include direct laser writing, microcontact printing,20,78 stamp-assisted printing,21 and tape nanolithography.5,18,24,26 Merits of transfer printing via laser induction or shellac relative to these alternatives are (1) preservation of the nanoscale order, (2) minimal operational requirements, and (3) compatibility with living tissues. Direct laser writing requires high laser power and vacuum conditions as reported by, e.g., Bagheri et al.,14 Beliatis et al.,79 Lee et al.,16 and others.14,79-83 These are incompatible with living tissues. Microcontact printing uses reactive precursors or conductive heating, as reported by Cerf et al.20 and Vohra et al.,78 which are also tissue-incompatible. Stamp-assisted printing transfers microscale structures using adhesive backing, as reported by Chen et al.,21 and transfer to tissue has not been reported. Various approaches to tape nanolithography by Ketelson et al., Oren et al., Shi et al., Wang, L. et al., Wang, Q., and others require conductive heating to 70 °C and/or residual adhesive incompatible with tissues.5,7,18,24,26 Overall, transfer printing via laser induction or shellac preserved the nanoscale order and a high yield of transferred nanoparticles sufficient to support a LSR that could improve sensing. The transfer was conducted at ambient conditions with a low or no power requirement. Transfer to A. cannabinum indicated the biocompatibility of each method.

4. CONCLUSIONS

Transfer printing by laser induction and shellac relocated AuNPs self-assembled into a square submicrometer lattice of cavities in PDMS onto glass and leaf cuticle substrates. Optical and laser scanning confocal microscopy showed that submicron lattice spacing was preserved in ≥90% of transfer-printed areas up to 4 mm2, and up to 90% of AuNP were transferred. This was sufficient to support the first surface lattice resonance from AuNPs transfer printed onto glass measured by transmission UV–vis spectroscopy. Preservation of coloration and AuNP order on transfer-printed A. cannabinum leaves indicated the biocompatibility of each method. Laser induction and shellac preserved the nanoscale order with minimal operational requirements for biocompatible transfer printing, showing the potential to improve sensing in flexible and transient electronic circuits.

Supplementary Material

Supporting information

ACKNOWLEDGMENTS

This research work was supported by the Center for Advanced Surface Engineering (CASE) under the National Science Foundation (NSF) Grant No. OIA-1457888, the Arkansas EPSCoR Program, and NIH R15 EY035066. The authors would like to acknowledge Megan Lanier for the fabrication of the bare PDMS stamps via soft lithography used for making ordered arrays for the transfer processes discussed in this work. Marvinsketch 23.14 was used to draw chemical structures (https://www.chemaxon.com).

Footnotes

Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.3c02700.

Transfer printing via laser induction and shellac, ImageJ analysis of AuNP occupancy and percent transfer, and biochemistry at the PDMS/AuNP–leaf cuticle interface that enabled transfer (PDF)

The authors declare no competing financial interest.

Contributor Information

Keith R. Berry, Jr., Nanocellutions LLC, Fayetteville, Arkansas 72701, United States; Division of Research and Innovation, University of Arkansas, Fayetteville, Arkansas 72701, United States.

Donald Keith Roper, Department of Biological Engineering, Utah State University, Logan, Utah 84322, United States.

Michelle A. Dopp, Ralph E. Martin Department of Chemical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States

John Moore, II, Nanocellutions LLC, Fayetteville, Arkansas 72701, United States.

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Supplementary Materials

Supporting information
NIHMS1999634-supplement-Supporting_information.pdf (1.3MB, pdf)

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