Mechanically sensitive molecules known as mechanophores have recently attracted much interest due to the need for mechanoresponsive materials. Maleimide–anthracene mechanophores located at the interface between poly(glycidyl methacrylate) (PGMA) polymer brushes and Si wafer surfaces were activated locally using atomic force microscopy (AFM) probes to deliver mechanical stimulation. Each individual maleimide–anthracene mechanophore exhibits binary behavior: undergoing a retro-[4 + 2] cycloaddition reaction under high load to form a surface-bound anthracene moiety and free PGMA or remaining unchanged if the load falls below the activation threshold. In the context of nanolithography, this behavior allows the high spatial selectivity required for the design and production of complex and hierarchical patterns with nanometer precision. The high spatial precision and control reported in this work brings us closer to molecular level programming of surface chemistry, with promising applications such as 3D nanoprinting, production of coatings, and composite materials that require nanopatterning or texture control as well as nanodevices and sensors for measuring mechanical stress and damage in situ

Liu, Gang-yu

Moore, Jeffrey S.

Robb, Maxwell J.

Sottos, Nancy R.

Sulkanen, Audrey R.

Sung, Jaeuk

Caltech Authors - Main

S1 
 
Supporting information for 
 
Spatially Selective and Density-Controlled Activation of Interfacial 
Mechanophores  
Audrey R. Sulkanen1,‡ Jaeuk Sung,2,4‡ Maxwell J. Robb,3,4,5 Jeffrey S. Moore,2,3,4 Nancy R 
Sottos2,4*, Gang-yu Liu1,* 
‡These authors contributed equally to this work. 
  
 
Table of Contents 
1. Materials ..................................................................................................................................... 2 
2. Confirmation of Mechanochemical Activation using Fluorescence Microscopy, ToF-SIMS, 
Optical Microscopy, and AFM Topographic Imaging ................................................................... 3 
3. Control Sample Results............................................................................................................... 4 
4. Production of Hierarchical Features Using AFM ....................................................................... 5 
5. Sample Fabrication Process ........................................................................................................ 6 
6. Imaging and Activation of Active and Control Samples .......................................................... 10 
7. Fluorescence Measurement ....................................................................................................... 12 
8.  ToF-SIMS Imaging .................................................................................................................. 12 
9. References for SI....................................................................................................................... 13 
 
 
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1. MATERIALS 
 
Polished silicon wafers, Si(100) with 300 nm thermal oxide layer, were purchased from 
University Wafers Inc. (Boston, MA, U.S.A.) for sample fabrication. Dimethyl sulfoxide 
(DMSO) (≥99.9%) was purchased from Sigma-Aldrich (St. Louis, MO, U.S.A.) and used 
without further purification. Ethanol (200 proof) was purchased from Sigma Aldrich. Water 
(≥18.2 MΩ) was purified by a Milli-Q system (Q-GARD 2, Millipore, Billerica, MA, U.S.A.). 
Polished silicon wafers, Si(111) doped with boron, were purchased from Virginia Semi- 
conductor Inc. (Fredericksburg, VA, U.S.A.) and used for cantilever calibration. Sulfuric acid 
(95.0%), hydrogen peroxide (30% aqueous solution) were purchased from Sigma-Aldritch (St. 
Louis, MO, U.S.A.). Nitrogen gas (99.999%) was purchased from Praxair, Inc. (Danbury, CT, 
U.S.A.). AC240TS-R3 silicon cantilevers were purchased from Oxford Instruments Asylum 
Research (6310 Hollister Ave, Santa Barbara, CA, 93117, U.S.A.). All other materials were used 
without further treatment or modification, unless otherwise stated. 
  
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2. CONFIRMATION OF MECHANOCHEMICAL ACTIVATION USING 
FLUORESCENCE MICROSCOPY, TOF-SIMS, OPTICAL MICROSCOPY, AND 
AFM TOPOGRAPHIC IMAGING 
 
 
Chemical and topographical changes after mechanochemical activation of interfacial MA at 
micrometer scale were confirmed with fluorescence microscopy, Time of Flight-Secondary Ion 
Mass Spectroscopy (ToF-SIMS), optical microscopy, and AFM. Shown in Figure S1A, the 
fluorescence image was collected from 410-430 nm emission upon 360 nm excitation, which 
detects fluorescence from the surface bound anthracene.1The fluorescence signal exclusively 
from the fabricated region confirms that high contact force successfully translated to 
mechanochemical activation. ToF-SIMS (Figure S1B) for the CNO- negative ion, originating 
from the maleimide fragment,2 was collected to map surface distribution of the intact MA adduct 
on the surface after fabrication. A low concentration of maleimide moiety was detected inside 
the fabricated ‘T’ feature, whereas the intact regions showed a relatively high concentration. This 
result is consistent with the fluorescence measurements that showed selective mechanophore 
activation where high contact force was applied, which led to the loss of the maleimide fragment. 
The optical micrograph in Figure S1C shows color contrast between the fabricated region and 
intact region due to removal of the polymer brush. AFM topography image shows height 
difference after fabricating the ‘T’ feature with 450 nN contact force, which further verifies the 
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removal of the PGMA brush.  Details of each experiment are provided below and in the main 
text. 
 
3. CONTROL SAMPLES  
 
To determine if PGMA removal was due to the mechanophore group, and to verify that the 
PGMA brush itself is stable under high force, we subjected a control sample (see Figure S2C) to 
the same 450 nN force with identical scan parameters as the mechanophore sample shown in 
Figure 2.  The control sample shows no height decrease after high force application (see Figure 
S2B), indicating none of the PGMA brushes were cleaved. Upon 360 nm excitation, the control 
sample showed no detectable fluorescence at 410-430 nm (Figure S2D), further verifying that the 
fluorescence seen in the mechanophore sample is due to the cleavage of the maleimide-
anthracene moiety.  
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4. PRODUCTION OF HIERARCHICAL PATTERNS 
 
The robustness of the spatially selective activation was further demonstrated by producing a 
hierarchical feature (Figure S3). Using the design tool included in the AFM’s software 
(MicroAngelo macro written using Igor Pro 6.34), the hierarchical structure was designed in two 
steps: first, the hollow letter ‘T’; second, periodical squares fill the space within the hollow ‘T’ 
feature. Upon setting the load (700 nN) and the physical dimensions, the AFM scan replicated 
the design in a couple of minutes. In the lateral force image shown above, the anthracene-termini 
region exhibited lower friction than surrounding polymer brush terminated by tert-butyl bromide, 
which is expected given the hydrophobic nature of the anthracene termini and the hydrophilic 
character of the cantilever. The grids lines of the ‘T’ feature were 83 nm wide. The inner grid 
rectangles measured 142 nm x 161 nm, while the size of the overall feature is 2.2 µm x 2.8 µm. 
This magnitude of difference in size indicates that this method is capable of producing small 
features without sacrificing the fidelity of the larger scale feature, verifying its robustness. The 
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fidelity of the feature produced coupled with the magnitude in difference in feature size verifies 
the robustness of the high activation of the MA mechanophore using AFM technology.  
5. SAMPLE FABRICATION PROCESS 
 
 
 
 
5.1 Active Specimen Fabrication 
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Active specimens with Maleimide-Anthracene (MA) mechanophores immobilized on the 
surface were prepared with a surface functionalization approach following previously reported 
methods, shown in Figure S4.3 In this work, the surface-bound MA mechanophores terminated 
with a bromoisobutyrate group were used to initiate a copper-catalyzed living radical 
polymerization of glycidyl methacrylate to grow polymer brushes.  
5.1.1 Surface Functionalization of Silicon Substrate with MA Mechanophore 
500 m thick silicon substrates with 300 nm thermally grown oxide layer were cleaned in 
piranha solution at 120 ℃ for 30 minutes. Cleaned substrates were washed with DI water and 
dried in a stream of air. The substrates were further dried in a convection oven at 120 °C for 30 
minutes. For surface functionalization, cleaned substrates were immersed in a 10 mM toluene 
solution of functionalized maleimide-anthracene adduct and kept in a sealed container for 24 
hours on a bench top. After 24 hours, the substrates were sonicated in toluene and subsequently 
rinsed with toluene, isopropyl alcohol, and DI water followed by drying under a stream of air. 
5.1.2 Surface Patterning MA Mechanophore Functionalized Silicon Substrate  
The patterned MA surface was fabricated by photo patterning a photoresist (AZ 5214 E, 
microChem) and removing exposed MA moieties with oxygen plasma (Harric Plasma Cleaner 
Pdc-32g)4. After oxygen plasma treatment, residual photoresist was removed by rinsing with N-
methyl-2-pyrrolidone.  
5.1.3 Polymer Brush Formation on MA Functionalized Substrate 
Poly(glycidyl methacrylate) brushes with varying thicknesses were synthesized on MA 
initiator-functionalized substrates using ARGET-ATRP.5 Silicon substrates with patterned MA 
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initiator were placed in 20 ml vial containing 2 ml methanol/DMF/anisole (1:1:1 volume ratio). 
To the vial, 1.7 g of glycidyl methacrylate (Sigma-Aldrich, filtered through basic alumina to 
remove inhibitor) and 2 ml of a catalyst stock solution (containing 0.0036 mmol CuBr2 and 
0.036 mmol PMDETA) were added. After mixing, the vial was purged with nitrogen for 20 
minutes. Ethyl-2-bromoisobutyrate (7 l, EIB) was added to simultaneously initiate solution 
polymerization of glycidyl methacrylate along with the surface-initiated polymerization. The 
molecular weight of free polymer EIB-PGMA was used as a reference to estimate the degree of 
polymerization of the surface attached polymer.6 The mixed solution was subjected to three 
cycles of freeze-pump-thaw process for complete degassing. After degassing, vial was filled with 
nitrogen and 1 ml ascorbic acid stock solution (8.4 mM ascorbic acid in methanol/DMF/anisole 
(1:1:1 volume ratio) solvent) was added. Four samples were prepared, which were polymerized 
for 10 minutes (three samples) and 20 minutes (one sample). After polymerization, the specimen 
was washed with DCM and ethanol. To remove residual solvent, we dried the silicon substrate in 
a vacuum oven at 50 ℃ for 24 hours. Representative size exclusion chromatography data is 
shown in Figure S6. The weight average molecular weight (Mw), number average molecular 
weight (Mn), and PDI of the synthesized polymer is summarized in Table S1. Thickness of the 
polymer brush was determined using AFM probe in DMSO, which were 11.4 ± 1.2 and 26.0 ± 
1.3 nm. For the sample containing the gradient feature, spiral pattern, and the hierarchical ‘T’ 
pattern, the exposed silicon surface after oxygen plasma etching was functionalized in 10 mM 2-
[methoxy(polyethyleneoxy)6-9propyl]trichlorosilane(oligomeric ethylene oxide) (Gelest) toluene 
solution.  For the samples with the passivated oligomeric ethylene oxide surfaces, the polymer 
brush heights were 9.2 ± 0.6 and 10.7 ± 1.3 nm relative to the surrounding oligomeric ethylene 
oxide.  The oligomeric ethylene oxide layer height was measured at 1.4 ± 0.2 nm. 
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Specimen Figures 2, 4, 5 Figure 3 
Polymerization Time 10 min 20 min 
Mw (kDa) 29.1 46.9 
Mn (kDa) 22.2 40.8 
PDI 1.31 1.15 
Brush Height 
11.4 ± 1.2 nm, 
10.6 ± 0.6 nm,  
12.1 ± 1.3 nm  
26.0 ± 1.3 
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5.2 Control Specimen Fabrications 
The control specimen (Figure S5) was fabricated to investigate the effects of high-load force on 
PGMA brush without the MA mechanophore. The control specimen was prepared using a similar 
fabrication steps to that of the active specimen. Piranha-cleaned silicon substrate was 
functionalized with (3-(trimethoxysilyl)propyl 2-bromo-2-methylpropionate, Gelest) by 
immersing it in 10 mM toluene solution for 24 hours. The functionalized surface was 
subsequently patterned using photolithography. PGMA brush was prepared by ARGET-ATRP 
(20 minute reaction time) as described above. The thickness of the polymer brush was 9.6 nm in 
DMSO, which was determined by AFM.      
 
6. IMAGING AND ACTIVATION OF ACTIVE AND CONTROL SAMPLES 
 
6.1 AFM Imaging  
All mechanophore and control samples were characterized using an atomic force microscope 
(MFP-3D, Asylum Research Corp., Santa Barbara, CA). Silicon probes, AC 240-TS (Olympus 
America, Central Valley, PA) were used for imaging and activation. The nominal force constant 
of the probes was 1.7 N/m, with a resonant frequency of 70 kHz in air. Silicon probes were used 
in their original state, with a brief cleaning in ethanol and nitrogen drying before each 
experiment. All experiments were carried out in DMSO in a liquid cell. Before imaging, all 
cantilevers were calibrated on a clean Si (111) wafers.  
 In the DMSO media, the mechanophore features were imaged in contact mode with a load of 
10-66 nN, with speeds ranging from 2.50-135.22 µm/s. The AFM images were acquired and 
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analyzed using Asylum MFP-3D software developed on the Igor Pro 6.34 platform. 
6.2 Preparation of Silicon Wafers for Calibration 
Polished silicon wafers were used to calibrate the cantilever before AFM imaging and 
activation. The wafers were cleaned by immersion in piranha solution for 30 minutes, and 
cleaned twice more with fresh piranha solution before being rinsed with copious amounts of 
milli-Q water. Cleaned wafers were subsequently stored in ultra-pure water, and rinsed with 
ethanol and dried under nitrogen before further use. 
6.3 AFM Activation 
Activation of the surface bound mechanophore was achieved using an atomic force microscope 
(MFP-3D, Asylum Research Corp., Santa Barbara, CA). The mechanophore samples were 
imaged under low forces [10-66 nN] until suitable areas were found. Silicon probes, AC 240-TS 
(Olympus America, Central Valley, PA) were used for activation by scanning with a high force 
(ranging from 200 nN to 1.0 µN) in contact mode in DMSO. After the high force scan, the areas 
were imaged again with low force scans to determine the extent of mechanophore activation.  
   6.4 AFM Custom Design Microlithography 
   Mechanophore regions were imaged at low contact forces (10-66 nN) using AFM to determine 
suitable areas for microlithography. Utilizing custom design software in Igor Pro 6.34, a bitmap 
image (either user designed or taken from the internet) was uploaded into the program, converted 
into greyscale and translated into force vectors. The color scale was assigned minimum and 
maximum force values, ranging from nanonewtons to micronewtons.  Additional parameters, 
such as feature size, scan speed, lines per scan and scan angle were also specified. Feature 
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fabrication using this method typically took anywhere from 2-8 minutes, depending on feature 
size, scan speed and image line density. After the lithography scan was completed, the area was 
imaged again under low force with the same AFM tip to assess success. 
 
7. FLUORESCENCE MEASUREMENT 
Fluorescence images were acquired using a Cascade 512b high sensitivity camera, which was 
attached to Zeiss Axiovert 200M. A mercury lamp source was used with 360 nm 
centered/FWHM 11 nm band pass excitation filter, 410 nm pass dichroic mirror, and 420 
nm/FWHM 20 nm band pass filter (Edmund Optics). For Figure 3 fluorescence measurements, 
the exposure time was set to 100 ms and 40x magnification on objective lens. Fluorescence 
images were processed with Image J.7 Fluorescence intensity was measured by averaging over 
20 m x 20 m region. Normalized photoluminescence (Figure 3 Right) was calculated by 
setting the average fluorescence intensity of 600 nN applied specimen to 100 and non-activated 
bare specimen as 0. For Figure 5 fluorescence measurements, the exposure time was 200 ms and 
63x magnification on objective lens. 
 
8. ToF-SIMS IMAGING 
Active specimens that were subjected to a contact force of 450 nN were analyzed with ToF-
SIMS (Physical Electronics PHI Trift III) imaging. For ToF-SIMS imaging, Au liquid source run 
with Au+ ion under static mode accelerated at 22 KeV energy was used as the source. Data was 
collected for 10 minute duration. 
 
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9. REFERENCES FOR SI  
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Promoting Mechanochemistry of Covalent Bonds by Noncovalent Micellar Aggregation Acs 
Macro Lett 2016, 5, 995-998. 
(2) Tischer, T.; Rodriguez-Emmenegger, C.; Trouillet, V.; Welle, A.; Schueler, V.; Mueller, J. 
O.; Goldmann, A. S.; Brynda, E.; Barner-Kowollik, C. Photo-Patterning of Non-Fouling 
Polymers and Biomolecules on Paper Adv. Mater. 2014, 26, 4087-4092. 
(3) Sung, J.; Robb, M. J.; White, S. R.; Moore, J. S.; Sottos, N. R. Interfacial Mechanophore 
Activation Using Laser-Induced Stress Waves J. Am. Chem. Soc. 2018, 140, 5000-5003. 
(4) Chen, J.-K.; Chang, C.-J. Fabrications and applications of stimulus-responsive polymer films 
and patterns on surfaces: a review Materials 2014, 7, 805-875. 
(5) Song, Y.; Ye, G.; Lu, Y.; Chen, J.; Wang, J.; Matyjaszewski, K. Surface-Initiated ARGET 
ATRP of Poly(Glycidyl Methacrylate) from Carbon Nanotubes via Bioinspired Catechol 
Chemistry for Efficient Adsorption of Uranium Ions ACS Macro Letters 2016, 5, 382-386. 
(6) Li, D.; Sheng, X.; Zhao, B. Environmentally responsive “hairy” nanoparticles: Mixed 
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Spatially Selective and Density-Controlled Activation of Interfacial Mechanophores

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Spatially Selective and Density-Controlled Activation of Interfacial Mechanophores

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