Grafting of Ring-Opened Cyclopropylamine Thin Films on Silicon (100

Subscriber access provided by UNIV OF ARIZONA

Article

Grafting of Ring-Opened Cyclopropylamine thin films on Silicon (100) Hydride via UV Photoionization Joline Tung, Jing Yuan Ching, Yoke Mooi Ng, Lih Shin Tew, and Yit Lung Khung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08343 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Grafting of Ring-Opened Cyclopropylamine thin films on Silicon (100) Hydride via UV Photoionization J. Tung1#, J. Y. Ching2#, Y. M. Ng3, L. S. Tew3 and Y. L Khung2*

1

College of Arts and Sciences, University of North Carolina at Chapel Hill (UNC) Chapel Hill, NC 27514, USA 2

Institute of New Drug Development, China Medical University No.91 Hsueh-Shih Road, Taichung, Taiwan 40402, R.O.C 3

Advanced Medical and Dental Institute (AMDI) Universiti Sains Malaysia, 13200 Kepala Batas Pulau Pinang Malaysia *

Corresponding Author: Dr. Yit Lung Khung Email: [email protected] #

Both authors had contributed equally in this work.

Abstract

The grafting of Cyclopropylamine onto silicon (100) hydride (Si-H) surface via ring opening mechanism using UV photoionization was described here. In brief, radicals generated from Si-H surface upon UV irradiation was found to behave in classical Hydrogen Abstraction Theory (HAT) manner by which the distal amine group was firstly hydrogen abstracted and the radical propagated down to the cyclopropane moiety. This subsequently liberated the strained bonds of the cyclopropane group and initiating the surface grafting process, producing a thin film of approximately 10-15 nm in height. Contact Angle (CA) measurements had also shown that such this photoionization had yield extremely hydrophilic surface (~17.5°) with extremely low hysteresis (160°C), the reaction of proceeded predominantly through the nucleophilic NH2 group to form a Si-N linkage to the surface. This rendered the surface hydrophobic and hence suggesting that Si-H homolysis model may not be the main process. To the best of the author’s knowledge, this was the first attempt in literature

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

using photoionization to directly graft cyclopropylamine onto silicon surface and in due course generating a highly rich NH terminated surface that was found to be highly bioactive in promoting cell viability based on MTT studies.

Keywords: Cyclopropylamine, UV photoionization, surface radical, X-ray Photoelectron Spectroscopy, MTT cytotoxicity assay.

Introduction Forming self-assembled monolayers on silicon surfaces via Si-C linkage had always been the focus in surface chemistry for a couple of important reasons. Firstly, Si-C bonds are extremely stable even under harsh acidic/basic conditions and are less likely to undergo degradation in many aqueous high salt environments1. While Si-O-X type organosilane linkages are generally more popular in literature due to its relative simplicity in experimental requirements, these chemical linkages may experience degradation issues as well as the poor suitability for electronic application considering that thin oxide layer is an insulating layer.

So far, Linford and Chidsey’s description of forming Si-C linkage on silicon surfaces via radicalized “dangling” Si bond in hydrosilylation process (UV or thermal) had been widely accepted within the scientific community for many years2-3. Yet in the recent decade, Hamer’s electron ejection4 or the exciton mechanism as described by Buriak5 had also offered alternative explanations other than the Si-H homolysis model. Currently, the general consensus is that UV or white light would generate a Si surface radical and its interaction with an unsaturated carbon would ultimately produce a stable Si-C bond on the surface2,

6-12

. Yet, the exact mechanism

governing thermal hydrosilylation remains argumentative and obscure6. Furthermore, both UV and thermal hydrosilylation suffer from certain limitations. Firstly, as Sieval et al. had stated that steric hindrance simply dictates the inability for all silicon atoms on the surface to be fully passivated13-14.

Secondly, the presence of nucleophilic groups in the grafting monomer,

especially at high temperatures, may adversely affect the outcome of the reaction due to their preference to bind to silicon-radicalized surface. This was well-described by Buriak et al. as


2

Page 3 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


well as several past papers from our group and this had certain restrictions on the choice of bifunctional molecules that can be grafted to the surface15-18.

In the classical hydrosilylation sense, unsaturated carbons such as alkenes and alkynes were usually used as initiating groups7. Therefore at first glance, the proposition of using quasi Sp2 type molecule, such as cyclopropyl-based compounds would seem rather illogical due to the lack of real π bonding systems from the cyclopropyl group as well as the instability of cyclopropane. However, the behavior of ring opening reactions of cyclopropyl-based molecule such as cyclopropylamine had been widely described, especially via radical based mediators19. In fact, it is the cyclopropylamine’s sensitivity towards radicals that results in such ring opening event 19-20 that may as well permit for a deeper scrutiny into the surface process during UV and thermal hydrosilylation. Coupled by the fact that cyclopropylamine does not absorb at the UV range, it would enable for careful examination of the outcome from radicals that are generated solely from the silicon surface during UV photoionization in the classical hydrosilylation setup. Another important reason to consider cyclopropylamine was its’ excellent mediator for bioactivating surfaces for biological application21-22

Herein, we describe and propose a simple process under UV photoionization by which cyclopropylamine would undergo an unique ring opening via hydrogen abstraction from its amine end from the silicon surface that subsequently result in the ring opening of the molecule (scheme 1). The radicalized form would consequently graft onto silicon to form Si-C bonds, producing a rich polymeric NHX surface. Moreover, UV photoionized cyclopropylamine grafted Si surface was also found to be an excellent promotor of cell viability from MTT cytotoxicity assays. However at higher temperatures (>160°C), we did not notice the effects of the Si-H homolysis as the nucleophilic groups have the preference to bind directly to silicon-radicalized surface15-18.

This report described, for the first time, the behavior of cyclopropylamine to

interfacing with silicon hydride surface under two principle conditions, thermal excitation and UV irradiation


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 37

Scheme 1. Graphical illustration of thermal reaction and UV Photoirradiation of Cyclopropylamine on silicon (100) hydride surface and the observed outcome as reported in this work.

Experimental Section Methods and Materials Boron-doped (100) silicon wafers, with resistivity of 0.001-0.005 Ω-cm were used in this experiment and was purchased from Semiconductor Wafer, Inc. (SWI). Unless otherwise stated, all reagents were purchased from Sigma-Aldrich and were used as received without further purification. Thermal initiated reaction Silicon wafers were cut approximately into 10 x 10 mm2 dimensions, and then immersed for 30 mins into a hot Pirahna solution (3 parts 95% Sulfuric acid and 1 part 34.5-36.5% hydrogen peroxide) to clean the surface. The wafer was dipped into an aqueous 5% hydrofluoric acid for 30 seconds. Afterwards, the sample was transferred to a degassed (minimum of 10 freeze-pumpthaw cycles) solution of 100 mM cyclopropylamine (in mesitylene which was stored in an argon environment. It was submerged into an oil bath at 160oC and reacted for approximately 18 hrs.


4

Page 5 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The surface was washed subsequently with mesitylene followed by sonication in chloroform and dichloromethane. All surfaces were kept under vacuum in a benchtop desiccator.

UV initiated reaction: The silicon wafers were prepped in a similar fashion to those in the thermal reaction. Following the HF dip, the sample was transferred onto a quartz disc and 50 ul of cyclopropylamine (neat) was added directly to the surface. A second quartz disc was placed on top (emulating a sandwich structure of the quartz disc-silicon wafer-quartz disc). The sample was illuminated using a UV light (UVP PenRay) that was placed 1 cm above to initiate the reaction for a duration of 120 mins. After the UV photonization, the surfaces were washed by sonication with methanol, ethanol and DCM in sequential order (1 min each) followed by a rinse with copious amounts of DI water before being blown dry with argon. The surfaces were finally stored in desiccator prior to surface analysis. Contact-angle measurements: A custom made goniometer was used to capture water contact-angle values on the samples. It was comprised of a CCD camera (SMN-B050-U (B/W)) that acquired the images at a resolution of 2560x1920. For each sessile droplet measurement, three separate 2 ul droplets were dispensed onto the selected sample and the drop images were recorded. Post-analysis of the droplet was done through Dropsnake 2.1. A total of 5 images were collected for each modification regime. X-ray Photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy analysis was performed on a PHI 5000 VersaProbe (ULVACPHI) equipped with an Al Kα X-ray source (1486.6 eV) and taken at an angle of 45° relative to the surface. Spectra were also obtained for the C1s, O1s, Si2p and N1s in high resolution for all samples. The spectra were subsequently analyzed and deconvoluted using XPSpeak while the atomic concentration was determined by CasaXPS (version 2.3.18)


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 37

Gold coating Au(OH)3 was prepared by mixing the aqueous solution of 4.5mL of 0.1M sodium hydroxide (NaOH) with 20mL of 6.35mM chloroauric acid (HAuCl4) thoroughly. After 15 minutes of stirring, the functionalized surfaces were then introduced into the mixture under 70°C for 24 hours. The surfaces were then washed with deionized water prior to immerse into 5mL of previous prepared K-gold solution (K-gold was prepared by stirring 1.5mL of 25mM HAuCl4 in 100mL of deionized water. The pH of K-gold was adjusted to 10.1 by addition 60mg of potassium carbonate (K2CO3) and allowed the solution to stir overnight at room temperature.). After that, 1mL of freshly prepared 6.6mM sodium borohydride (NaBH4) per 10mL of K-gold was introduced to begin the reducing process. Ten minutes later, the surfaces were then washed with deionised water and dried with argon.

Atomic Force Microscopy Atomic force microscopy (AFM) images were acquired on a Digital Instrument NS4/D3100CL/MultiMode Scanning Probe Microscope running on in-build AFM tapping mode with cantilever tuned at Freq. 150 kHz, Force 5 N/m in triplicates. Scan area on the surfaces were of 1 µm x 1 µm and the scan speed was set at 0.6 hz with the integral and proportional gain set at automatic mode. Post image processing was performed with Gwyddion MacOS version 2.38.

APTES grafted surface Unmodified silicon wafer was firstly cleaned by hot Pirahna solution for 30 mins in and then rinsed with copious amount of deionised water prior to silanization. The cleaned silicon wafer was then immersed in the solution of APTES (50 mM in toluene) at room temperature. After 2 hours, the silicon wafer was sonicated in methanol, ethanol followed by DCM for 1 minute respectively. APTES functionalized wafer was sterilised by immersion in 70% ethanol and exposure to UV light for 30 minutes.


6

Page 7 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Collagen grafted surface Cleaning of unmodified pristine silicon wafer was performed by first immersing in Piranha solution for 30 mins. The silicon wafer was then rinsed thoroughly with deionised water and then blown dry with argon. After that, 200 µl of 0.1% collagen was added on silicon surface and the reaction was left for 30 minutes. Upon completion, the excess fluid was removed and the coated surface was air-dried at room temperature. The coated surface was eventually sterilised by exposure to UV light for 30 minutes in the cell-culture laminar hood prior to cell seeding.

Poly-L-lysine Firstly, unmodified silicon wafer was washed by sonication in methanol, ethanol followed by dichloromethane. This wafer will then blow-dried with argon. Next, poly-L-lysine was added to the silicon wafer and incubated for 30 minutes at room temperature. After that, the coated surface was allowed to dry in the cell-culture laminar hood and sterilised by exposure to UV light before introducing of cells.

Cell culture All cell cultures were incubated in 5% CO2 incubator at 37o C. MDA-MB-231 cells were cultured in RPMI supplemented with 10% fetal bovine serum and 1% penicillin- streptomycin. The silicon surfaces are placed in 24-well plate and seeded with an initial density of 3x104 cells. After 24 hours, the cells medium was removed and washed three times with PBS followed by fixation using 4% paraformaldehyde for 20 mins. Afterwards, the solution was dispensed and the surfaces were rinsed with 1x PBS before proceeding with permeabilization of the cells using 0.1% Triton X-100 in PBS. A 1µl stock solution of phalloidin-555 (AAT Bioquest) was diluted with 100ul of PBS containing 1% bovine serum as working solution to stain cells actin filaments. To ensure no excess Phalloidin remained, the surfaces were also washed with PBS three times followed by staining of the nuclei with 1 µl of Hoechst 33342 (AAT Bioquest) in 1 ml of PBS buffer for 10 mins. Upon completion of staining, a drop of Fluoromount (NOVUS) was added to


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 37

the surface and was then covered with a coverslip and subsequently sealed with nail varnish. To perform the cell count analysis, a total of n=5 surfaces (10 mm by 10 mm each) were studied for each conditions selected and 5 random spots were chosen on each of the surface. Fluorescence microscopy images were taken for each of the random spots and the number of cells was subsequently quantified by counting. All data were tabulated and their standard derivations were calculated accordingly from Excel spreadsheet.

Cell viability (MTT assay) All cell cultures were incubated in 5% CO2 incubator at 37o C. MDA-MB-231 cells were cultured in RPMI supplemented with 10% fetal bovine serum and 1% penicillin- streptomycin. The various silicon surfaces (10 mm by 10 mm) are placed in 24-well plate and seeded with an initial density of 3x104 cells. After 24 hours, the surfaces were gently moved into a new 24-well plate and 200 µl of RPMI medium was added into each well along with 40 ul of Assay Solution (Cell Meter Colorimetric Cell Cytotoxicity Assay Kit purchased from AAT Bioquest). The solutions were mixed by gently shaking the 24-well plate for 30 seconds. The surfaces were then incubated at 37o C, 5% CO2 incubator for 4 hours. After the incubation, the absorbance change in the 24-well plate was observed at 570 nm and 605 nm in a multiplate reader. The ratio of OD570 to OD605 is used to determine the cell viability in each well. The readings of 5 replicates were consolidated and the values were normalized at 100% in conjunction to those from the unmodified control (piranha cleaned) silicon surfaces.

Results Two sets of experimental conditions were selected to examine the behavior of cyclopropylamine on silicon (100) hydride surface, namely (1) thermally reacted at temperature above 160°C and (2) UV photoionization. Prior to diving into the results, it is important to reiterate a couple of hypotheses here. Firstly, the cyclopropane moieties in cyclopropylamine was not expected absorb at the UV range as the photon energy at 254 nm (4.88 eV) was insufficient to radicalize the cyclopropylamine’s distonic amine as described by Bouchoux et al23. Furthermore, there was


8

Page 9 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


no aromatic acceptor to improve on the surface reactivity8. Hence all mechanistic explanation could only find its explanation solely from the emergence of silicon surface radicals, may it be through Chidsey’s Si-H homolysis model2-3, Hamer’s electron ejection model4 or those as described by Buriak5. Secondly, based on our previous studies, it was to be expected that if there was to be any reaction to the surface, it should be predominantly localized from the distonic amine to form initial Si-N bonding due to its nucleophilic nature. However, upon the completion of the two reactions, it was immediately apparent that the surfaces had, in actuality, reacted rather differently.

Figure 1. Contact angle measures on (a) pristine unmodified surface, (b) thermally reacted and (c) UV photoionized surfaces. The AFM images for (d) thermal reacted surface and (e) UV photoionized surface had shown a different cross section profile (f and g respectively)

To compare among the surfaces, unmodified silicon surfaces (control) had been first cleaned in piranha solution followed by storage in a desiccator prior to analysis. After the reaction, contact angle measurement were performed on both the thermal and UV surfaces. All representative


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 37

survey spectrums were as shown in supplementary figure S1-3. As shown in figure 1b, thermal reaction on cyclopropylamine had exhibited a wettability profile of 70.8° ± 5.3° while UV photoionized surface had a relatively low contact angle at 21.3° ± 1.9°. While there was little notable difference between the unmodified surfaces (66.7° ± 5.7°) with the thermally reacted surfaces, the extensive lowering of the wettability profile by UV photoionization had been rather unusual. AFM profiling of the surface had also shown that thermal reaction had profiled a relatively thin film of ~0.25 nm that was barely detectable and was consistent to the height of a Si-N linked cyclopropylamine generated from MM2 minimized iteration (see figure 1f inset). On the other hand, UV photonization had produced a thin polymeric film of approximately between 10-15 nm (figure 1e) as revealed by the cross-section profiling (although it is important to state that this particular AFM analysis had been performed along a deliberately excised line made gently running a pair of sharp tweezers across the surface). This was done to in order to make an exact evaluation of the film thickness from AFM. To understand the grafting on the surface as a function of time, UV photoionization was performed at various time points and AFM was used to examine these surfaces. As shown in figure 2, the film thickening could be easily visualised over the period of 120 mins and in order to evaluate the stability of these grafting, a 120 min sample was subjected to harsh basic conditions (PBS buffer adjusted to pH 9) for an hour of incubation. From the AFM analysis, it was noticed that subjecting that film to basic conditions could have resulted in a slight lowering of overall surface coverage. However, we also found that even with the subjection of the thin film to such harsh basic conditions at 1 hour was not sufficiently detrimental to reduce the overall bioactivity of the material (see MTT section).


10

Page 11 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. AFM analysis of film formation via UV photoionization of Cyclopropylamine at different time points. (a) unmodified silicon surface, (b) 15 mins of grafting, (c) 30 mins of grafting, (d) 60 mins of grafting and (e) 120 mins of grafting. In conjunction to this, upon 120 mins of grafting, the surface ( was subjected to shaking incubation for an hour in PBS buffer that adjusted to a pH of 9 (f) to evaluate the stability of the film.

Several observations could be made from the XPS examination of the survey spectrum as well as the atomic concentration as determined from CASAXPS software. Firstly, it was noticeable that the oxygen level for the unmodified surface was higher (42.77 ± 3.5%) compared to both the thermally reacted (31.80 ± 5.1%) and UV photoionized (23.67 ± 2.6%) (See table 1 below). This has strongly indicated that the hydride surface had been better passivated by cyclopropylamine under the UV photoionization reaction conditions than compared to the thermal reactions. The richer concentration of carbon for UV photoionization (52.61 ± 6.1%) had further helped to substantiate this claim. Furthemore, we notice a reduction in silicon percentage for UV photoionization and this may be consistent with the presence of a thin polymeric film on the


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

surface of the substrate. Although it must stated that the standard derivation for nitrogen concentration was relatively high but in view of the uneven grafting observed in AFM, this result may corroborate with each other.

Atomic Concentration (%) Si2p

O1s

N1s

C1s

Unmodified silicon

39.32 ± 2.8

42.77 ± 3.5

0.025 ± 0.05

18.88 ± 2.22

Thermal excitation

36.31 ± 4.3

31.80 ± 5.1

0.70 ± 0.17

31.19 ± 4.1

UV photoionization

17.24 ± 4.8

23.67 ± 2.6

6.48 ± 2.1

52.61 ± 6.1

Table 1. Atomic concentration on silicon (100) surface undergoing various reaction conditions with cyclopropylamine at n=5.

Inspection of the Si2p line had revealed a very unusual reaction behavior for the cyclopropylamine. As shown in figure 2, the first observation was the subdued Si 2p3/2 (99 eV) and Si 2p1/2 (99.5 eV) for the thermal excitation (blue) when compared to the UV photoionized surface (orange). More importantly was the emergence of dominant peak centering at 102.1 eV for the thermally excited surface and this peak was taken as indicative of a Si-N type linkage on the surface, as illustrated by Wiggins et al from their work on thermal nitridation on silicon surfaces24. In parallel with our previous work on Si-O-C type linkage16, a conjugation of nitrogen to silicon was thought to induce a slight shift to the right when compared to Si-O linkages and this was in part due to the exerting of lower electronegativity demands from the silicon atom. Hence this assignment for 102.1 eV as Si-N deemed logical. A lack of unsaturated carbon from the cyclopropylamine should suggest that all reaction should only proceed from the nucleophilic NH2 in the thermal reaction and this was in agreement with our observations of the XPS. However, what was interesting was that UV photoionized surface did not follow such reaction route other than the usual lowering of oxidation level as compared to the unmodified silicon.

In view of this trend, a closer examination into the Si2p spectrum for the UV

photoionized surface had uncovered some peculiarities at the region from 100-102 eV. A Gaussian-Lorentzian curvefit at 100.4eV was attempted for the Si-C25-27 and much to our surprise, it was actually possible to accommodate for this peak without compromising the overall


12

Page 13 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fit.

However, considering the difficulties of assigning Si-C at that position due to the

coexistence of the large resident Si 2p1/2 peak, especially for thin films27-28, it was thought to be prudent not to forcefully argued that Si-C linkage on the surface was the only outcome of the reaction process. Additionally, the notion that some of the silicon atoms on the surface would react via the NH2 had help served against making such statement at this junction.

Figure 3. XPS Si2p spectrum for the unmodified silicon (100) surface (green), UV photoionized (red) and the thermally excitation (blue) of cyclopropylamine on silicon hydride surface.

The possible emergence of the Si-C linkage for the cyclopropylamine from the Si2p spectrum remained perplexing due to primary assumption that all reactivity should only commence via the NH2 site based on many studies in the past. This “quasi” sp2 bond of the cyclopropane region was not thought to possess sufficient impetus to react directly to Si-H surface during UV photoionization. To better understand the nature of the grafted molecule to the surface, C1s and N1s analysis was also examined. Confirmation of the Si-C on the UV photonized surface (figure 3b) could be immediately visualized from a notable notch at the main carbon shouldering and a peak could be fitted at 283.5 eV that was indicative of this bond29-30. Examination of the C1s on both surfaces had firstly displayed the typical Sp2 C=C at 284.0 eV31-32 and an Sp3 type C-C at 284.6-284.7 eV33. Much effort was taken to deconvolute both Sp2 and Sp3 C-C, in view of


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

certain peculiarities observed in the C1s spectrum, namely the broad FWHM of the nominal C-C peak as well as notable asymmetrical peak presentation. The ability to accommodate both hydrization states had shed much light on the nature the surface grafting, suggesting that a ring opening could have occurred. The presence of C-N at 285.8 eV and 286.0 eV34-36 for both the thermal and UV photonized surface respectively was confirmed on the surface although decovolution into its various hybridization states (Sp3 C-N and Sp2 C=N) remained challenging at most cases35. Nonetheless, the large FWHM of the C-N peak for both samples had strongly suggested for a viable presence of C=N group. Upon consultation with previous studies on cyclopropylamine radical clock ring opening mechanism, terminal imine C=N was often reported as an outcome and thus our observations was in found to be in agreement19, 23. However, more interesting was the emergence of the peak at ~288.1 eV for both surface that was normally assigned to the transition π* of N-C=O bonding or a C-C=O type functionality37-39. How this could have occurred may be due to the presence of oxygen that was radicalized during the course of thermal and UV photoionization treatment. But the occurrence with the thermally excited reaction was unexpected due to the stringent deoxygenation process utilized. Nonetheless, the detail description of this process would be provided in the discussion section of the manuscript.


14

Page 15 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. XPS C1s spectrum for the thermally excited (a) and the UV photoionization (b) of cyclopropylamine on silicon hydride surfaces. The appearance of the Si-C bonding centering 283.8 could be fitted to the spectra for the UV photonionized surface (see circle inset in 3b). In conjunction, the deconvolution N1s for Thermal excitation (c) and UV photoionization (d) was also performed and to illustrate the presence of the Si-N bond nature for thermally excited surfaces.

Evidence of the direct reaction of cyclopropylamine to silicon hydride surface and its subsequent ring opening (if any) was immediate made from N1s analysis. The major difference in N1s between the thermal reaction (figure 4c) and UV photoionization (figure 4d) was the emergence of the distinctive peaks for Si-N at 397.7 eV40-41 and 400.1 eV42-43 for the thermally reacted cyclopropylamine (figure 4c). Radi et al. had described on the behavior of ethanolamine thermally reacting to silicon (100) surface44 and had found that the Si-N bond had also the tendency of forming Si-N-Si bridges. On the basis of this argument, we prudently fitted and


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

assigned the peak centering at 399.1 eV as Si-N-Si bridging. Although it was important to state that while their described reaction mechanism may have differ from ours, dangling bond was the common precursor and this had let us to conclude that radical transfer upon interfacing with the silicon could have lead to Si-N-Si bridging situation. Hence, the reaction mechanism underlying thermal excitation was far richer than first anticipated. Regarding the peak centering at 402.2, it was very common to find in literature assignment of this peak as a secondary or quaternary bind nitrogen45-47, an artifact that arises from the direct interaction between silicon and cyclopropylamine. Such trend was also as reported by Lange et al48. On the contrary, no notable Si-N peak could be fitted on the UV photoionized surface (figure 4d). The broad peak from 398.8 eV may suggest a strong presence of imine (-C=N) on the surface49, a resultant from a ring opening of cyclopropylamine as previously reported19.

Hence, from the N1s spectrum, it was possible to see that the major linkage formed to the surface was one of Si-N nature for the thermally reacted surface and that was not the case for the UV photoionized surface. O1s was also examine for the nature of oxygen binding to the two surface and was found to be relatively similar, with peaks centering at 532.1 eV (see figure S4). But in view that this peak was reported to be representative of both Si-O and O=C-N, discerning the respective composition may not technically fesasible. As shown below (table 2) are all the peak assignments as well as FWHM for this work.


16

Page 17 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. List of assignments as reported in this work together with the individual FWHM as well as calculated area.

Furthermore, in conjunction to XPS, we also performed ATR-FTIR on the thermal as well as UV photoionized surfaces. As shown in Figure S5, the most notable feature was the presence of the deformation from 1300-1400 cm-1 for the thermal reacted surfaces while this was clearly absent in the UV photoionized surfaces. This was attributed to the C-H deformation bending from cyclopropane ring system and was therefore indicative of the non ring-opening in thermal


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 37

reactions. However, considering the issues of sensitivity as well as film thickness in our work, FTIR could only be used to complement existing XPS data.

To demonstrate the applicability of a cyclopropylamine passivated silicon surface, gold nanoseed was deposited on the silicon (100) surface prepared via thermal excitation and UV photoionization of cyclopropylamine as well as an unmodified silicon surface. Gold deposition on the these surface was another alternative to determine and demonstrate the overall surface coverage on the surface. This is due to the fact that silicon hydroxide (Si-OH) was deemed to be too negatively charged to promote gold binding while any distonic NH group would conferred a positive charge that can in turn encourage gold grafting. As shown in Figure S6, AFM analysis of unmodified silicon surface had revealed a very little gold deposition (Figure S6a) while UV photoionized surface was found to have the best overall gold coverage (Figure S6c). Thermally reacted surface with cyclopropylamine could be described to fall in between both ends of the spectrum in terms of gold coverage (Figure S6b).

Figure 5. (a) Fluorescence microscopy and merge overlay images of MDA-MB 231 cells at 10x magnification growing after 24 hours various surface conditions. (b) MTT cytotoxicity assay was also performed to evaluate the cell biocompatibility of these surfaces and their respective values were as shown in (c).


18

Page 19 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Considerable strides have been made towards passivating silicon surfaces with amines in the past due to its biocompatibility and our work had the potential to lead to the same outcome. MDAMB-231 cells typically exhibit elongated cell adhesion morphology; hence, observing their adhesion on the surface was a clear indicator of biocompatibility. As shown in Figure S8, MDAMB 231 cells were seeded on various surfaces and incubated for duration of 24 hours. The control chosen was piranha treated silicon surface generating rich Si-OH termination on the surface. After 24 hours, both control and thermal surfaces was found to be poor in promoting cell adhesion (36.9 ± 9.96 cells/mm2 and 22.05 ± 6.66 cells/mm2 respectively). On the other hand, UV photoionized surfaces (149.70 ± 12.69 cells/mm2) compared favourably with the more popular APTES silanized surfaces (178.51 ± 9.35 cells/mm2). It was obvious that collagen had the best-obtained results (268.95 ± 19.47 cells/mm2). Interestingly while poly-L-lysine coating had presented a fairly comparable adhesion number (133.05 ± 7.32 cells/mm2) but the 40x magnification (Figure S7d) had shown that the adhering cells were mostly round in morphology. More importantly, the UV grafted surfaces were also subjected to degradation by incubation in PBS buffer adjusted to pH 9 for 30 and 60 mins in a shaker incubator. Although there was a slight reduction in cell count after 30 mins and 60 mins of incubation (113.85 ± 15.05 and 106.95 ± 15.09 cells/mm2 respectively), the surface remained relative attractive towards cell adhesion in general.

When compared to the control, the cell count was still found to be statistically

significant (P value of ≤0.05).

In order to validate the surface cytotoxicity, MTT cytotoxicity assay was performed on the various surfaces and as shown in figure 5, the trends were highly identical to those from our cell count studies. At first glance, both the UV and the APTES surface share relatively similar MTT values (167.69% ± 3.51% and 170.73 ± 5.02). If these values were taken in consideration with the cell count studies, one could conclude that while APTES had seemingly promote more cells adhesion, the overall metabolic activity for APTES was lower compared to the UV grafting. However, more interesting was that the MTT values for both the samples undergoing the pH 9 degradation (143.39% ± 8.57% and 146.15% ± 20.15%) had still shown a higher MTT reading compared to the poly-L-Lysine surfaces (120.27% ± 12.03%). The cell count for Lysine coated surface may have been higher but in view of the ‘roundish’ morphology as observed in the fluorescence imaging for the Lysine coated surface, it may be argued that cell adhering on the


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

Lysine coated surfaces may not be metabolically active than compared to our UV grafted surfaces. Considering that Lysine had often been deploy in laboratories to produce bioactive surfaces for cell adhesion, our results here may have demonstrated the potential of using UV photoionization of cyclopropylamine-based thin film as another feasible alternative on silicon surfaces.

Discussion While the outcome of the thermal reaction reaction was relatively straightforward in forming SiN linkage, the chemistry underlying was very rich. From the XPS analysis, it was certain that the thermal reaction had mainly proceeded via the NH2 group and that majority of the surface had been passivated with a Si-N linkage with the distal end being that of cyclopropane group. This was consistent to what had been proposed from literature50-53 as well as our observations from XPS examination. In conjunction, we were able to deconvolute for the presence of a Si-NSi bridge and this was in full agreement with the report by Bergerson et al.52. In view of this, we had attempted to rationalize a more plausible step-by-step reaction mechanism (Scheme 1a). To begin with, we felt unwise to rely on the notion of surface radicalization via Si-H homolysis at high temperature due to many reports on the possibility of surface grafting even at lower temperatures ( 160°C).

It was reported here that by means of surface

radicalization, it was possible to destabilized a strain-ring cyclopropane moiety and the outcome was quite unexpected ranging from the detection of Si-C linkage to subsequent grafting of a rich amidated polymer under UV photoionization. These surfaces were used to graft gold layers as well as perform cell adhesion/cytotoxicity studies and the results had shown that the end result of this unique UV reaction has produced an NHx rich terminated silicon surface. Furthermore, MTT studies had revealed that the film was highly comparable to standard grafting methods such as APTES and Poly-L-Lysine in terms of promoting metabolic activity on surfaces. Even after extensive incubation in basic environment, these surfaces were found to be still bioactive in terms of promoting cell viability. In summary, in the course of understanding the reactivity of cyclopropylamine under various reaction conditions, we had stumbled onto a molecule candidate that enables for the grafting rich bioactive NHx surface. To the best of the authors’ knowledge, this was the first time cyclopropylamine was grafted covalently to silicon hydride surface and many of the results obtained here could unravel the potential of using such saturated but straincompromised molecule for direct passivation to silicon surface, thus offering another model to the classical silicon hydrosilylation with alkene/alkyne.

Supporting Information Supporting information contains the following; XPS survey spectrum and O1s spectrum for unmodified silicon surface, thermal reacted and UV photoionized surfaces. ATR-FTIR analysis as well as AFM for gold passivation on silicon surface had been provided. Cell count data as well as cell morphology observation via fluorescence microscopy was also included. AUTHOR INFORMATION Corresponding Author


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

Dr. Yit Lung Khung, PhD, MRSC Email: [email protected] Address: Institute of New Drug Development, China Medical University, No.91 Hsueh-Shih Road, Taichung, Taiwan 40402, R.O.C

Author Contributions J. Tung, J. Y Ching and Y.M. Ng had performed the surface modifications while J. Tung had contributed much to the XPS surface analysis. J. Y. Ching had performed all MTT bioactivity studies as well as the time course studies on the film formation. J. Y. Ching and L.S. Tew had also performed the cell cultures while L.S. Tew had worked on the gold coating. Y.L. Khung had conceived the project concept as well as having written this majority of this manuscript.

ACKNOWLEDGMENT

The work was carried out with assistance from university grant (CMU-1055918D) and grant under Ministry of Science and Technology in Taiwan (MOST 105-2218-E-039-002)

REFERENCES 1. Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D., Alkyl Monolayers on Silicon prepared from 1-Alkenes and Hydrogen-Terminated Silicon. J. Am. Chem. Soc. 1995, 117 (11), 3145-3155. 2. Linford, M. R.; Chidsey, C. E. D., Alkyl Monolayers covalently bonded to Silicon Surfaces. J. Am. Chem. Soc. 1993, 115 (26), 12631-12632. 3. Wade, C. P.; Chidsey, C. E. D., Etch-pit Initiation by Dissolved Oxygen on Terraces of H-Si(111). App. Phys. Lett. 1997, 71 (12), 1679-1681. 4. Wang, X. Y.; Ruther, R. E.; Streifer, J. A.; Hamers, R. J., UV-Induced Grafting of Alkenes to Silicon Surfaces: Photoemission versus Excitons. J. Am. Chem. Soc. 2010, 132 (12), 4048-+. 5. Buriak, J. M., Illuminating Silicon Surface Hydrosilylation: An Unexpected Plurality of Mechanisms. Chem. Mater. 2014, 26 (1), 763-772.


24

Page 25 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6. Ciampi, S.; Harper, J. B.; Gooding, J. J., Wet Chemical Routes to the Assembly of Organic Monolayers on Silicon Surfaces via the Formation of Si-C bonds: Surface Preparation, Passivation and Functionalization. Chem. Soc. Rev. 2010, 39 (6), 2158-2183. 7. Ng, A.; Ciampi, S.; James, M.; Harper, J. B.; Gooding, J. J., Comparing the Reactivity of Alkynes and Alkenes on Silicon (100) Surfaces. Langmuir 2009, 25 (24), 13934-13941. 8. Huck, L. A.; Buriak, J. M., UV-Initiated Hydrosilylation on Hydrogen-Terminated Silicon (111): Rate Coefficient Increase of Two Orders of Magnitude in the Presence of Aromatic Electron Acceptors. Langmuir 2012, 28 (47), 16285-16293. 9. Stewart, M. P.; Buriak, J. M., Photopatterned Hydrosilylation on Porous Silicon. Angewandte Chemie-International Edition 1998, 37 (23), 3257-3260. 10. Buriak, J. M., Silicon-Carbon Bonds on Porous Silicon Surfaces. Advanced Materials 1999, 11 (3), 265-+. 11. Boukherroub, R.; Morin, S.; Bensebaa, F.; Wayner, D. D. M., New Synthetic Routes to Alkyl Monolayers on the Si(111) Surface. Langmuir 1999, 15 (11), 3831-3835. 12. Boukherroub, R.; Wayner, D. D. M.; Sproule, G. I.; Lockwood, D. J.; Canham, L. T., Stability Enhancement of Partially-Oxidized Porous Silicon Nanostructures Modified with Ethyl Undecylenate. Nano Lett. 2001, 1 (12), 713-717. 13. Sieval, A. B.; Linke, R.; Heij, G.; Meijer, G.; Zuilhof, H.; Sudholter, E. J. R., AminoTerminated Organic Monolayers on Hydrogen-Terminated Silicon Surfaces. Langmuir 2001, 17 (24), 7554-7559. 14. Sieval, A. B.; van den Hout, B.; Zuilhof, H.; Sudholter, E. J. R., Molecular Modeling of Covalently Attached Alkyl Monolayers an the Hydrogen-Terminated Si(111) Surface. Langmuir 2001, 17 (7), 2172-2181. 15. Khung, Y. L.; Ngalim, S. H.; Meda, L.; Narducci, D., Preferential Formation of Si-O-C over Si-C Linkage upon Thermal Grafting on Hydrogen-Terminated Silicon (111). Chem. Eur. J. 2014, 20 (46), 15151–15158. 16. Khung, Y. L.; Ngalim, S. H.; Scaccabarozi, A.; Narducci, D., Thermal and UV Hydrosilylation of Alcohol-Based Bifunctional Alkynes on Si (111) surfaces: How Surface Radicals Influence Surface Bond Formation. Sci. Rep. 2015, 5, 1-13 17. Khung, Y. L.; Ngalim, S. H.; Scaccabarozzi, A.; Narducci, D., Formation of Stable Si-OC Submonolayers on Hydrogen-Terminated Silicon(111) under Low-Temperature Conditions. Beilstein. J. Nanotech. 2015, 6, 19-26. 18. Buriak, J. M., Organometallic Chemistry on Silicon Surfaces: Formation of Functional Monolayers bound through Si-C bonds. Chem. Comm. 1999, (12), 1051-1060. 19. Nonhebel, D. C., The Chemistry of Cyclopropylmethyl and Related Radicals. Chem. Soc. Rev. 1993, 22 (5), 347-359. 20. Zhang, X. Q.; Li, X. X.; Liu, Y. F.; Wang, Y., Suicide Inhibition of Cytochrome P450 Enzymes by Cyclopropylamines via a Ring-Opening Mechanism: Proton-Coupled Electron Transfer Makes a Difference. Front. Chem. 2017, 5, 1-10. 21. Strbkova, L.; Manakhov, A.; Zajickova, L.; Stoica, A.; Vesely, P.; Chmelik, R., The Adhesion of Normal Human Dermal Fibroblasts to the Cyclopropylamine Plasma Polymers studied by Holographic Microscopy. Sur. Coat. Tech. 2016, 295, 70-77. 22. Manakhov, A.; Makhneva, E.; Skladal, P.; Necas, D.; Cechal, J.; Kalina, L.; Elias, M.; Zajickova, L., The Robust Bio-immobilization Based on Pulsed Plasma Polymerization of Cyclopropylamine and Glutaraldehyde Coupling Chemistry. App. Sur. Sci. 2016, 360, 28-36.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

23. Bouchoux, G.; Alcaraz, C.; Dutuit, O.; Nguyen, M. T., Unimolecular Chemistry of the Gaseous Cyclopropylamine Radical Cation. J. Am. Chem. Soc. 1998, 120 (1), 152-160. 24. Wiggins, M. D.; Baird, R. J.; Wynblatt, P., Thermal Nitridation of Si(111) by Nitric Oxide. J. Vac. Sci. Tech. 1981, 18 (3), 965-970. 25. Gomez, F. J.; Prieto, P.; Elizalde, E.; Piqueras, J., SiGN Alloys Deposited by Electron Cyclotron Resonance Plasma Chemical Vapor Deposition. App. Phys. Lett. 1996, 69 (6), 773775. 26. Wang, Y. X.; Wen, J.; Guo, Z.; Tang, Y. Q.; Tang, H. G.; Wu, J. X., The Preparation of Single-Crystal 4H-SiC Film by Pulsed XeCl Laser Deposition. Thin Solid Films 1999, 338 (1-2), 93-99. 27. Masuda, T.; Iwasaka, A.; Takagishi, H.; Shimoda, T., Properties of Phosphorus-Doped Silicon-Rich Amorphous Silicon Carbide Film Prepared by a Solution Process. J. Am. Ceramic. Soc. 2016, 99 (5), 1651-1656. 28. Wang, Y. H.; Lin, J.; Huan, C. H. A., Multiphase Structure of Hydrogenated Amorphous Silicon Carbide Thin Films. Mater. Sci. Eng. B 2002, 95 (1), 43-50. 29. Coletti, C.; Emtsev, K. V.; Zakharov, A. A.; Ouisse, T.; Chaussende, D.; Starke, U., Large Area Quasi-Free Standing Monolayer Graphene on 3C-SiC(111). App. Phys. Lett. 2011, 99 (8), 1-3. 30. Tsai, H. S.; Hsiao, C. H.; Chen, C. W.; Ouyang, H.; Liang, J. H., Synthesis of Nonepitaxial Multilayer Silicene Assisted by Ion Implantation. Nanoscale 2016, 8 (18), 94889492. 31. Zuo, Z.; Xu, Z. G.; Zheng, R. J.; Khanaki, A.; Zheng, J. G.; Liu, J. L., In-situ Epitaxial Growth of Graphene/h-BN van der Waals Heterostructures by Molecular Beam Epitaxy. Sci. Rep. 2015, 5, 1-6. 32. Tang, Z. W.; Chen, X. W.; Chen, H.; Wu, L. M.; Yu, X. B., Metal-Free Catalysis of Ammonia-Borane Dehydrogenation/Regeneration for a Highly Efficient and Facilely Recyclable Hydrogen-Storage Material. Angew. Chem. Int. Ed. 2013, 52 (22), 5832-5835. 33. Biniak, S.; Szymanski, G.; Siedlewski, J.; Swiatkowski, A., The Characterization of Activated Carbons with Oxygen and Nitrogen Surface Groups. Carbon 1997, 35 (12), 17991810. 34. Swiatowska-Mrowiecka, J.; Zanna, S.; Ogle, K.; Marcus, P., Adsorption of 1,2Diaminoethane on ZnO Thin Films from p-xylene. App. Sur. Sci. 2008, 254 (17), 5530-5539. 35. Permatasari, F. A.; Aimon, A. H.; Iskandar, F.; Ogi, T.; Okuyama, K., Role of C-N Configurations in the Photoluminescence of Graphene Quantum Dots Synthesized by a Hydrothermal Route. Sci. Rep. 2016, 6, 1-8. 36. Schweiger, B.; Kim, J.; Kim, Y. J.; Ulbricht, M., Electropolymerized Molecularly Imprinted Polypyrrole Film for Sensing of Clofibric Acid. Sensors 2015, 15 (3), 4870-4889. 37. Tyan, Y. C.; Liao, J. D.; Jong, S. B.; Liao, P. C.; Yang, M. H.; Chang, Y. W.; Klauser, R.; Himmelhaus, M.; Grunze, M., Characterization of Trypsin Immobilized on the Functionable Alkylthiolate Self-Assembled Monolayers: A Preliminary Application for Trypsin Digestion Chip on Protein Identification using Matrix-Assisted Laser Desorption/Ionization Time-of-Flight mass spectrometry. J. Mater. Sci. Mater. Med. 2005, 16 (2), 135-142. 38. Yu, D. S.; Dai, L. M., Self-Assembled Graphene/Carbon Nanotube Hybrid Films for Supercapacitors. J. Phys. Chem. Lett. 2010, 1 (2), 467-470.


26

Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


39. Zhang, L. B.; Wang, J. Q.; Wang, H. G.; Xu, Y.; Wang, Z. F.; Li, Z. P.; Mi, Y. J.; Yang, S. R., Preparation, Mechanical and Thermal Properties of Functionalized Graphene/Polyimide Nanocomposites. Compos. Part A-Appl. S. 2012, 43 (9), 1537-1545. 40. Yan, X. B.; Tay, B. K.; Chen, G.; Yang, S. R., Synthesis of Silicon Carbide Nitride Nanocomposite Films by a Simple Electrochemical Method. Electrochem. Comm. 2006, 8 (5), 737-740. 41. Dwivedi, N.; Rismani-Yazdi, E.; Yeo, R. J.; Goohpattader, P. S.; Satyanarayana, N.; Srinivasan, N.; Druz, B.; Tripathy, S.; Bhatia, C. S., Probing the Role of an Atomically Thin SiNx Interlayer on the Structure of Ultrathin Carbon Films. Sci. Rep. 2014, 4, 1-10. 42. Karcher, R.; Ley, L.; Johnson, R. L., Electronic Structure of Hydrogenated and Unhydrogenated Amorphous SiNx (0≤x≤1.6): A Photoemission Study. Phys. Rev. B 1984, 30 (4), 1896-1910. 43. Moon, C. S.; Takeda, K.; Takashima, S.; Sekine, M.; Setsuhara, Y.; Shiratani, M.; Hori, M., Surface Loss Probabilities of H and N radicals on Different Materials in Afterglow Plasmas employing H-2 and N-2 Mixture Gases. J. Appl. Phys. 2010, 107 (10), 1-7. 44. Radi, A.; Leung, K. T., Competitive Bonding of Amino and Hydroxyl Groups in Ethanolamine on Si(100)2 x 1: Temperature-Dependent X-Ray Photoemission and Thermal Desorption Studies of Nanochemistry of a Double-Chelating Agent. Mater. Express 2011, 1 (2), 144-153. 45. Zhou, M.; Yang, C. Z.; Chan, K. Y., Structuring Porous Iron-Nitrogen-Doped Carbon in a Core/Shell Geometry for the Oxygen Reduction Reaction. Adv. Eng. Mater 2014, 4 (18), 1-5. 46. Yuan, X. X.; Hu, X. X.; Ding, X. L.; Kong, H. C.; Sha, H. D.; Lin, H.; Wen, W.; Shen, G. X.; Guo, Z.; Ma, Z. F.; Yang, Y., Effects of Cobalt Precursor on Pyrolyzed Carbon-Supported Cobalt-Polypyrrole as Electrocatalyst toward Oxygen Reduction Reaction. Nanoscale Res. Lett. 2013, 8, 1-11. 47. Diller, K.; Klappenberger, F.; Marschall, M.; Hermann, K.; Nefedov, A.; Woll, C.; Barth, J. V., Self-Metalation of 2H-Tetraphenylporphyrin on Cu(111): An X-ray Spectroscopy study. J. Chem. Phys. 2012, 136 (1), 1-13. 48. Lange, N.; Dietrich, P. M.; Lippitz, A.; Kulak, N.; Unger, W. E. S., New Azidation Methods for the Functionalization of Silicon Nitride and Application in Copper-Catalyzed Azide-Alkyne Cycloaddition (CuAAC). Sur. Interface Anal. 2016, 48 (7), 621-625. 49. Xu, Z.-X.; Li, T.; Zhong, Z.-M.; Zha, D.-S.; Wu, S.-H.; Liu, Q.; Xiao, W.-D.; Jiang, X.R.; Zhang, X.-X.; Chen, J.-T., Amide-Linkage Formed Between Ammonia Plasma Treated Poly(D,L-lactide acid) Scaffolds and Bio-Peptides: Enhancement of Cell Adhesion and Osteogenic Differentiation In Vitro. Biopolymers 2011, 95 (10), 682-694. 50. Qiao, M. H.; Cao, Y.; Deng, J. F.; Xu, G. Q., Formation of Covalent Si-N linkages on Pyrrole Functionalized Si(100)-(2 x 1). Chem. Phys. Lett. 2000, 325 (5-6), 508-512. 51. Mui, C.; Wang, G. T.; Bent, S. F.; Musgrave, C. B., Reactions of Methylamines at the Si(100)-2x1 Surface. J. Chem. Phys. 2001, 114 (22), 10170-10180. 52. Bergerson, W. F.; Mulder, J. A.; Hsung, R. P.; Zhu, X. Y., Assembly of Organic Molecules on Silicon Surfaces via the Si-N linkage. J. Am. Chem. Soc. 1999, 121 (2), 454-455. 53. Kugler, T.; Thibaut, U.; Abraham, M.; Folkers, G.; Gopel, W., Chemically Modified Semiconductor Surfaces: 1,4-phenylenediamine on Si(100). Sur. Sci. 1992, 260 (1-3), 64-74. 54. Coletti, C.; Marrone, A.; Giorgi, G.; Sgamellotti, A.; Cerofolini, G.; Re, N., Nonradical Mechanisms for the Uncatalyzed Thermal Functionalization of Silicon surfaces by Alkenes and Alkynes: A Fensity Gunctional Study. Langmuir 2006, 22 (24), 9949-9956.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

55. Yu, Y. X.; Hessel, C. M.; Bogart, T. D.; Panthani, M. G.; Rasch, M. R.; Korgel, B. A., Room Temperature Hydrosilylation of Silicon Nanocrystals with Bifunctional Terminal Alkenes. Langmuir 2013, 29 (5), 1533-1540. 56. Zhu, X. Y.; Mulder, J. A.; Bergerson, W. F., Chemical Vapor Deposition of Organic Monolayers on Si(100) via Si-N linkages. Langmuir 1999, 15 (23), 8147-8154. 57. Tong, X.; DiLabio, G. A.; Clarkin, O. J.; Wolkow, R. A., Ring-Opening Radical Clock Reactions for Hybrid Organic-Silicon Surface Nanostructures: A New Self-Directed Growth Mechanism and Kinetic Insights. Nano Lett. 2004, 4 (2), 357-360. 58. Manakhov, A.; Zajickova, L.; Elias, M.; Cechal, J.; Polcak, J.; Hnilica, J.; Bittnerova, S.; Necas, D., Optimization of Cyclopropylamine Plasma Polymerization toward Enhanced Layer Stability in Contact with Water. Plasma Process. Polym. 2014, 11 (6), 532-544. 59. Rueping, M.; Vila, C.; Koenigs, R. M.; Poscharny, K.; Fabry, D. C., Dual Catalysis: combining Photoredox and Lewis Base Catalysis for Direct Mannich Reactions. Chem. Comm. 2011, 47 (8), 2360-2362. 60. Kohls, P.; Jadhav, D.; Pandey, G.; Reiser, O., Visible Light Photoredox Catalysis: Generation and Addition of N-Aryltetrahydroisoquinoline-Derived alpha-Amino Radicals to Michael Acceptors. Org. Letts. 2012, 14 (3), 672-675. 61. Hu, J.; Wang, J.; Nguyen, T. H.; Zheng, N., The Chemistry of Amine Radical Cations produced by Visible Light Photoredox Catalysis. Beilstein. J. Org. Chem. 2013, 9, 1977-2001. 62. Tucker, J. W.; Stephenson, C. R. J., Shining Light on Photoredox Catalysis: Theory and Synthetic Applications. J. Org. Chem 2012, 77 (4), 1617-1622.


28

Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Content

Grafting of Ring-Opened Cyclopropylamine thin films on Silicon (100) Hydride via Surface Radicals J. Tung, J. Y. Ching, Y. M. Ng, L. S. Tew * and Y. L Khung

Graphical illustration of thermal reaction and UV photoirradiation of cyclopropylamine on silicon (100) hydride surface and the observed outcome as reported in this paper.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 30 of 37

Page 31 of 37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 32 of 37

Page 33 of 37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 34 of 37

Page 35 of 37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 36 of 37

Page 37 of 37


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Grafting of Ring-Opened Cyclopropylamine Thin Films on Silicon (100

Recommend Documents