Article pubs.acs.org/Langmuir
Tracking Invisible Transformations of Physisorbed Monolayers: LDITOF and MALDI-TOF Mass Spectrometry as Complements to STM Imaging Jian He, Chen Fang, Russell A. Shelp,† and Matthew B. Zimmt* Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States S Supporting Information *
ABSTRACT: Triphenyleneethynylene (TPEE) derivatives bearing one long aliphatic chain on each terminal aryl ring and two short aliphatic chains on the central aryl ring (core chains) self-assemble single component and 1-D patterned, two-component, crystalline monolayers at the solution− graphite interface. The monolayer morphology directs the core chains off the graphite, making them accessible for chemical reactions but invisible to imaging by scanning tunneling microscopy (STM). This precludes using STM to monitor transformations of the core chains, either by reaction or solution−monolayer exchange of TPEE molecules. Laser desorption/ionization time-of-flight mass spectrometry (LDI-TOF MS) successfully identifies TPEE compounds within physisorbed monolayers. The LDI-TOF spectra of TPEE monolayer− graphite samples exhibit strong molecular ion peaks and minimal fragmentation or background. LDI-TOF and STM techniques are combined to evaluate monolayer composition and morphology, track solution−monolayer exchange, to identify reaction products and to measure kinetics of chemical reactions at the solution−monolayer interface. LDI-TOF MS provides rapid qualitative evaluation of monolayer composition across a graphite substrate. Challenges to quantitative composition evaluation by LDI-TOF include compound-specific light absorption, surface desorption/ionization and fragmentation characteristics. For some, but not all, compounds, applying matrix onto a self-assembled monolayer increases molecular ion intensities and affords more accurate assessment of monolayer composition via matrix assisted laser desorption/ionization (MALDI) MS. Matrix addition precludes subsequent chemical or STM studies of the monolayer, whereas reactions and STM may be performed at nonirradiated regions following LDI-TOF measurements. LDI- and MALDI-TOF MS are useful complements to STM and are easily implemented tools for study of physisorbed monolayers.
■
INTRODUCTION The imaging capability of scanning tunneling microscopy (STM) has played a critical role in efforts to increase patterning complexity of self-assembled monolayers.1−3 STM image analysis connects monolayer packing and morphology to the detailed structures and interactions of the molecular components. Efforts to use patterned monolayers as reactive templates pose new challenges to STM based analyses. STM can image “rigid” monolayer regions effectively but provides little information about mobile regions, e.g., at domain interfaces or of molecular components whose positions are constrained weakly by the monolayer.4−7 This precludes using STM to detect or to track reactions and processes that modify relatively mobile monolayer components.8,9 Methods that have been used to sense monolayer modifications include electrochemical STM,10 surface plasmon resonance,11 quartz crystal microbalance,12,13 Raman and infrared spectroscopy13,14 and XPS. 13,14 Over the last two decades, powerful mass spectrometry tools have been applied to analyze monolayers chemically bonded to surfaces: direct analysis in real time mass spectrometry (DART-MS)15 has been used to characterize monolayer composition16 and surface reaction kinetics17 under © 2016 American Chemical Society
ambient conditions; matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has been used to study self-assembly processes,18 chemical reactions19 and biological processes20,21 of monolayers bonded to metal surfaces (SAMDI22). Use of DART, MALDI-TOF, or LDITOF MS to study the self-assembly and reactions of monolayers physisorbed on highly oriented pyrolytic graphite (HOPG) or on other conductive surfaces is uncommon23 despite the use of graphite,24,25 other forms of carbon26−29 and other surfaces30−33 to enhance LDI-TOF signals. In this paper, LDI-TOF and MALDI-TOF MS are used to characterize reactions and self-assembly processes of physisorbed monolayers on HOPG that are not detectable by STM. Molecular ions are the dominant LDI-TOF MS signals detected from many of the triphenyleneethynylene monolayers34−38 investigated. LDI-TOF spectra and STM images can be collected from a monolayer−graphite sample both before and after reactions or other transformations are performed. For some Received: September 3, 2016 Revised: November 17, 2016 Published: December 19, 2016 459
DOI: 10.1021/acs.langmuir.6b03252 Langmuir 2017, 33, 459−467
Article
Langmuir
(least focused, 60 μm diameter) of the instrument’s SmartbeamTM-II Nd:YAG laser (355 nm) operating at 1 kHz. Data was collected in positive ion mode using 500 laser shots at 45−60% of full laser power, a TOF accelerating voltage of 19.45 kV, a grid voltage of 18.1 kV and pulsed ion extraction time of 90 μs. Instrument calibration for HOPG within the pressure mounted cell was performed using Bruker Daltonics Peptide Calibration Standard II (part no. 8222570 see Table SI-1.) Compound Syntheses and Characterization. See the Supporting Information.
physisorbed monolayers, applying a thin matrix layer enhances intensities of molecular ion peaks and the quantitative accuracy of monolayer composition analyses. STM imaging and monolayer reactions are not possible once matrix has been added. Both matrix free LDI-TOF and MALDI-TOF mass spectrometry are useful complements to STM for studies and applications of physisorbed monolayers.
■
EXPERIMENTAL SECTION
■
Monolayer Self-Assembly and Processing on HOPG. Drop cast monolayers on HOPG were prepared by applying 3 μL of 25 or 12.5 μM triphenyleneethynylene compound, T, in phenyloctane solutions to freshly cleaved 12 mm × 12 mm HOPG surfaces (ZYB grade, Momentive Performance, Strongsville, OH). After sitting for 15−30 min, monolayers were imaged by STM. To remove applied solution and nonadsorbed compounds, the HOPG substrate was held at a 20−25° angle from the horizontal and 20−100 μL of rinse solvent, typically phenyloctane, ethanol, tetrahydrofuran, or tetradecane, was applied along the raised HOPG edge and removed using an absorbent Kim Wipe in contact with the lower edge. HOPG Incubation for Monolayer Reactions. Aluminum or stainless steel blocks for monolayer reactions contained a square depression, 14 mm × 14 mm × 0.05 mm, in the flat bottom of a square, 17 mm wide chamber. A solution reservoir cut into the center of the depression was either 9 mm wide (square) and 0.25 mm deep (20 μL) for use with 20−30 μL of incubation solution, or 6 mm wide (circular) and 0.2 mm deep (6 μL) for use with 6−10 μL of incubation solution. Reagent solution was added to the reservoir and the HOPG substrate was placed, monolayer face down, into the square depression. Solution from the reservoir contacted the HOPG substrate and spread across its surface. A tight fitting lid was used to seal the block for incubations. The internal volume of the empty, sealed chamber was ∼800 μL. After incubation, the HOPG face bearing the monolayer was rinsed as described above. HOPG Mounting for Mass Spectrometry Studies. Two mounts were used to hold HOPG substrates for mass spectrometry analyses (see Figure SI-1). For precise mass determinations, an HOPG substrate bearing a physisorbed monolayer was pressure mounted into a cell with a 9 × 9 mm2 opening. The cell was screwed into a rectangular depression milled in a steel MTP PAC frame (Bruker, part no. 221598). This positions the monolayer bearing surface of any thickness HOPG substrate at the same height relative to the MALDI target plane, allowing precise mass calibration. For reaction studies, an HOPG substrate was attached to a 14 mm × 12 mm × 0.8 mm steel plate using copper conductive tape (Ted Pella, prod. no. 16074). A second copper tape held the HOPG substrate/steel plate (a stack) within one of eight rectangular wells, 15 mm × 15 mm × (0.4 + n × 0.2) mm deep (n = 0−7), milled into the MTP PAC frame. A rectangular well was selected so that the monolayer was positioned as close as possible to the MALDI target plane; if the HOPG top surface is lower than the target plane, the TOF determined mass is too heavy, with a mass shift of +1 amu (Δm/z) for every ∼100 μm increase of flight path (0.5−1.5 kDa range). Peeling HOPG to obtain clean surfaces alters the substrate thickness; this precludes exact alignment of the monolayer on top of the stack with the target plane. STM and Mass Spectrometry Procedures. STM images were acquired using a Digital Instruments NanoScope NS3A controller (Bruker, Camarillo, CA) interfaced to an HD-0.5I scan head. STM tips were cut from 80/20 Pt/Ir wire (0.25 mm, Goodfellow, Oakdale, PA). Monolayers on HOPG were prepared as described above. The STM tip was engaged through the applied solvent and scanned in constant current mode, occasionally in constant height mode. STM data was collected using set currents of 35−150 pA and bias voltages in the range −0.8 to −1.6 V. LDI-TOF and MALDI-TOF spectra of monolayers on HOPG were acquired using a Bruker Daltonics Autoflex MALDI-TOF mass spectrometer (Billerica, MA) running in linear mode. Sample holder (MTP PAC Frame) modifications to hold the HOPG substrate are described above. Samples were irradiated using the 6_ultra laser profile
RESULTS AND DISCUSSION STM, LDI-TOF MS, and Surface Exchange Studies of Triphenyleneethynylene Monolayers. Monolayers of [17]2T(OC6)2 (Chart 1) on HOPG were prepared by drop Chart 1. Line Structure and Abbreviations of the Triphenyleneethynylene (TPEE) Molecules Used in This Studya
The triphenyleneethynylene unit is abbreviated as “T”. “Side chains,” attached at the para-positions of the two terminal aryl rings, are identified in square brackets, e.g., hexadecyloxy “side chains” are abbreviated as [17]. “Core chains,” attached to the central aryl ring, are para to each other and are identified in parentheses, e.g., a hexyloxy chain is abbreviated as (OC6).
a
casting 3 μL of 25 μM phenyloctane solution at 19 °C. The molecules assemble crystalline domains (Figure 1a,b) in which triphenyleneethynylene (TPEE) columns (brighter) alternate with hexadecyloxy “side chain” ([17]) columns (darker). The monolayer unit cell contains one molecule and covers 3.7 nm2 (a = 3.7 ± 0.2 nm, b = 1.1 ± 0.1 nm, α = 69 ± 2 o). The b-axis spacing of TPEE cores is determined, primarily, by close packing of the interdigitated [17] side chains. This spacing does not provide room for an entire hexyloxy “core chain” (OC6) to contact the HOPG surface (vide infra). Each core chain terminus extends above the monolayer and has some mobility. The hexyloxy core chains, while not identifiable within STM images, likely contribute to the diffuse appearance of the TPEE cores (Figure 1a). LDI-TOF mass spectra (Figure 1c) were recorded from the STM sample after rinsing the HOPG substrate with 20 μL of phenyloctane to remove any applied [17]2T(OC6)2 not physisorbed to HOPG. The mass spectrum is remarkably simple; the [17]2T(OC6)2 molecular ion at m/z = 958.87 (958.78 calculated) is the dominant (base) peak and exhibits an isotopic pattern consistent with the composition (see Figure SI-2). Low intensity fragments at m/z = 873.72 and 887.83 have been assigned tentatively (see Figure SI-2). Monolayers on HOPG were prepared by drop casting [17]2T(OC6)(OC2OH), a TPEE derivative bearing one hexyloxy core chain and one 2-hydroxyethoxy core chain 460
DOI: 10.1021/acs.langmuir.6b03252 Langmuir 2017, 33, 459−467
Article
Langmuir
Figure 1. (a) 20 × 20 nm2 constant current STM image (0.12 nA, −0.85 V) of a monolayer assembled from 25 μM [17]2T(OC6)2 in phenyloctane. (b) 100 × 100 nm2 constant height STM image (0.035 nA, −0.85 V) of a monolayer assembled from 25 μM [17]2T(OC6)2 in phenyloctane. (c) LDI-TOF MS spectrum collected from a [17]2T(OC6)2 monolayer physisorbed on HOPG, 500 shots; inset: expansion of molecular ion region (m/z = 959) displaying isotopic pattern. Figure 2. (a) 20 × 20 nm2 STM constant current STM image (0.05 nA, −1.0 V) of a monolayer assembled on HOPG from 25 μM [17]2T(OC6)(OC2OH) in phenyloctane. (b) 150 × 150 nm2 constant current STM image (0.035 nA, −1.0 V) of a monolayer assembled from 13 μM [17]2T(OC6)(OC2OH) in phenyloctane. (c) LDI-TOF MS spectrum of [17]2T(OC6)(OC2OH) monolayer physisorbed on HOPG, 500 shots; inset: expansion of molecular ion region (m/z = 919). (d) LDI-TOF MS spectrum of [17]2T(OC6)(OC2OH) monolayer on HOPG after 48 h incubation in 6 μL of 12.5 μM [17]2T(OC6)2 phenyloctane; inset: twofold magnified intensity scale showing molecular ions of [17]2T(OC6)(OC2OH) (off scale) and of [17]2T(OC6) 2 (m/z = 959) that exchanged into the monolayer.
(OC2OH). STM images of [17]2T(OC6)(OC2OH) monolayers (Figure 2a,b) are not reliably distinguishable from STM images of [17]2T(OC6)2 monolayers (Figure 1a,b). LDI-TOF spectra (Figure 2c) collected from [17]2T(OC6)(OC2OH) monolayers after phenyloctane rinses exhibit an intense molecular ion peak at m/z = 918.65 (918.71 calculated) along with low intensity fragments at m/z = 833.51 and 847.65 (see Figure SI-3). In contrast to STM images, LDI-TOF spectra allow unequivocal differentiation and identification of [17]2T(OC6)(OC2OH) and [17]2T(OC6)2 in monolayers physisorbed on HOPG. The ability to differentiate TPEE analogs by LDI-TOF MS allows investigation of compound exchange15 in monolayers at the solution−HOPG interface. A monolayer of [17]2T(OC6)(OC2OH) on HOPG (drop cast: 3 μL of 25 μM phenyloctane solution) was rinsed with 50 μL of tetrahydrofuran, air-dried and inserted, monolayer side down, in a reaction chamber charged with 6 μL of 12.5 μM [17]2T(OC6)2 in phenyloctane. This amount of solvent wet the entire HOPG surface bearing the physisorbed monolayer. After 48 h at 19 °C, the HOPG was removed from solution and rinsed with 50 μL of tetrahydrofuran. LDI-TOF spectra collected from the HOPG substrate (Figure 2d) contained molecular ion peaks of [17]2T(OC6)(OC2OH) and [17]2T(OC6)2 with area ratios averaging 9:1 at sites sampled across the HOPG substrate. As both TPEE molecular ions fragment minimally and exhibit relative peak areas that closely track monolayer composition (vide infra), the LDI-TOF spectra indicate that [17]2T(OC6)(OC2OH) is the principal component of the post incubation monolayer, and that [17]2TPEE molecules in a monolayer exchange slowly (greater than tens of hours) into phenyloctane solution at 19 °C. Thus, it should be possible to chemically react physisorbed [17]2TPEE monolayers at the solution - HOPG interface. Nucleophilic Core Chains: Esterification and Reaction Kinetics of [17]2T(OC6)(OC2OH) Monolayers. [17]2T(OC6)2 and [17]2T(OC6)(OC2OH) assemble similar morphol-
ogy monolayers despite their different core chain structures (OC6 vs OC2OH). The distances between nearest TPEE cores are similar within both compounds’ monolayers. Molecular mechanics simulations of [17]2T(OC6)2 monolayers (Figure 3) with interdigitated, close packed [17] side chains produce intercolumn and intracolumn TPEE spacings in reasonable agreement with STM images. These simulations reveal insufficient empty space for each (OC6) “core” chain to adsorb fully to the underlying HOPG surface; the core chains lose
Figure 3. CPK model of a molecular mechanics minimized [17]2T(OC6)2 monolayer section on a graphene layer (green atoms). (a) Top view and (b) side view showing (OC6) core chains on top of adjacent TPEE cores. 461
DOI: 10.1021/acs.langmuir.6b03252 Langmuir 2017, 33, 459−467
Article
Langmuir
with irradiating LDI laser power, suggesting a “two-photon” origin of the “[M + 4]+” peak. The structure of this two photon ion was not determined. To probe whether [17]2T(OC6)(OC2OH) esterification occurs on the surface or in tetradecane solution, a [17]2T(OC6)(OC2OH) monolayer was prepared on HOPG (substrate #1) and rinsed as described above. A second HOPG substrate (#2) bearing a [17]2T(OC6)2 monolayer was prepared and rinsed. The two HOPG substrates were immersed, monolayer sides down and with edges in contact, in the same 25 μL of tetradecane containing 25 mM propanoic anhydride and 2.5 mM DMAP. Following 1 h incubation, LDITOF spectra of HOPG #1 revealed dominant peaks from the [17]2T(OC6)(OC2O(CO)C2) ester product, no peaks indicative of [17]2T(OC6)(OC2OH) starting material and very weak intensity peaks from [17]2T(OC6)2 (2500 μm2) is much larger than areas probed easily by STM. Combined studies could be used to investigate monolayer heterogeneity by comparing local (STM) and global (LDI-TOF) compositions. Desorption/ionization cross sections depend on monolayer components’ molecular and electronic structures, and on properties of the illumination source. Consequently, quantitative evaluation of monolayer composition requires prior determination of different components’ relative LDI cross sections or use of internal standards compatible with the monolayer. For some physisorbed monolayers, applying a thin layer of MALDI matrix increases molecular ion intensities and reduces, but does not eliminate,
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Matthew B. Zimmt: 0000-0003-3584-9859 Present Address †
R.A.S.: Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104. Author Contributions
All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was supported, in part, by National Science Foundation Grants CHE1058241 and CHE1607273. Assistance implementing the mass spectrometry experiments from Dr. Tun-Li Shen, Ken Talbot, Randy Goulet, and Ondřej Fejfar (Czech Technical University) is acknowledged gratefully.
■
ABBREVIATIONS LDI, laser desorption/ionization; MALDI, matrix assisted laser desorption/ionization; TOF, time-of-flight; MS, mass spectrometry; STM, scanning tunneling microscopy; HOPG, highly oriented pyrolytic graphite 465
DOI: 10.1021/acs.langmuir.6b03252 Langmuir 2017, 33, 459−467
Article
Langmuir
■
(19) Su, J.; Mrksich, M. Using MALDI-TOF mass spectrometry to characterize interfacial reactions on self-assembled monolayers. Langmuir 2003, 19, 4867−4870. (20) Su, J.; Mrksich, M. Using mass spectrometry to characterize selfassembled monolayers presenting peptides, proteins, and carbohydrates. Angew. Chem., Int. Ed. 2002, 41, 4715−4718. (21) Sodji, Q. H.; Patil, V.; Kornacki, J. R.; Mrksich, M.; Oyelere, A. K. J. Med. Chem. 2013, 56, 9969−99811. (22) Mrksich, M. Mass Spectrometry of Self-Assembled Monolayers: A New Tool for Molecular Surface Science. ACS Nano 2008, 2, 7−18. (23) Iverson, C. P. The assembly of planar and square-pyramidal metalloporphyrin monolayers on highly oriented pyrolytic graphite. Ph.D. Dissertation, University of Chicago, Chicago, IL, March 2012. (24) Sunner, J.; Dratz, E.; Chen, Y. C. Graphite surface assisted laser desorption/ionization time-of-flight mass-spectrometry of peptides and proteins from liquid solutions. Anal. Chem. 1995, 67, 4335−4342. (25) Chen, Y. C.; Wu, J. Y. Analysis of small organics on planar silica surfaces using surface-assisted laser desorption/ionization mass spectrometry. Rapid Commun. Mass Spectrom. 2001, 15, 1899−1903. (26) Xu, S. Y.; Li, Y. F.; Zou, H. F.; Qiu, J. S.; Guo, Z.; Guo, B. C. Carbon nanotubes as assisted matrix for laser desorption/ionization time-of-flight mass spectrometry. Anal. Chem. 2003, 75, 6191−6195. (27) Tang, H.-W.; Ng, K.-M.; Lu, W.; Che, C.-M. Ion desorption efficiency and internal energy transfer in carbon-based surface-assisted laser desorption/ionization mass spectrometry: desorption mechanism(s) and the design of SALDI Substrates. Anal. Chem. 2009, 81, 4720−4729. (28) Najam-ul-Haq, M.; Rainer, M.; Szabo, Z.; Vallant, R.; Huck, C. W.; Bonn, G. K. Role of carbon nano-materials in the analysis of biological materials by laser desorption/ionization-mass spectrometry. J. Biochem. Biophys. Methods 2007, 70, 319−328. (29) Zhang, H.; Cha, S.; Yeung, E. S. Colloidal graphite-assisted laser desorption/ionization MS and MSn of small molecules. 2. Direct profiling and MS imaging of small metabolites from fruits. Anal. Chem. 2007, 79, 6575−6584. (30) Peterson, D. S. Matrix-free methods for laser desorption/ ionization mass spectrometry. Mass Spectrom. Rev. 2007, 26, 19−34. (31) Finkel, N. H.; Prevo, B. G.; Velev, O. D.; He, L. Ordered silicon nanocavity arrays in surface-assisted desorption. Anal. Chem. 2005, 77, 1088−1095. (32) Law, K. P.; Larkin, J. R. Recent advances in SALDI-MS techniques and their chemical and bioanalytical applications. Anal. Bioanal. Chem. 2011, 399, 2597−2622. (33) Castellana, E. T.; Gamez, R. C.; Russell, D. H. Label-free biosensing with lipid-functionalized gold nanorods. J. Am. Chem. Soc. 2011, 133, 4182−4185. (34) Weigelt, S.; Busse, C.; Bombis, C.; Knudsen, M. M.; Gothelf, K. V.; Strunskus, T.; Wöll, C.; Dahlbom, M.; Hammer, B.; Lægsgaard; Besenbacher, F.; Linderoth, T. R. Covalent interlinking of an aldehyde and an amine on a Au(111) surface in ultrahigh vacuum. Angew. Chem., Int. Ed. 2007, 46, 9227−9230. (35) Ramesh, A. R.; Thomas, K. G. Directional hydrogen bonding controlled 2D self-organization of phenyleneethynylenes: from linear assembly to rectangular network. Chem. Commun. 2010, 46, 3457− 3459. (36) Mu, Z.; Yang, X.; Wang, Z.; Zhang, X.; Zhao, J.; Bo, Z. Influence of substituents on two-dimensional ordering of oligo(phenyleneethynylene)s-A scanning tunneling microscopy study. Langmuir 2004, 20, 8892−8896. (37) Jenny, N. M.; Wang, H.; Neuburger, M.; Fuchs, H.; Chi, L.; Mayor, M. Synthesis and solid-state investigations of oligo-phenylene− ethynylene structures with halide end-groups. Eur. J. Org. Chem. 2012, 2012, 2738−2747. (38) Ma, Z.; Li, Y.-B.; Deng, K.; Lei, S.-B.; Wang, Y.-Y.; Wang, P.; Yang, Y. L.; Wang, C.; Huang, W. Supramolecular assemblies of tetrahydroxyloligo(phenyleneethynylene) with cross-shaped side chains and its coadsorption with diacids on graphite. J. Phys. Chem. C 2010, 114, 11460−11465.
REFERENCES
(1) Elemans, J. A. A. W.; Lei, S.; De Feyter, S. Molecular and Supramolecular Networks on Surfaces: From Two-Dimensional Crystal Engineering to Reactivity. Angew. Chem., Int. Ed. 2009, 48, 7298−7332. (2) Lackinger, M.; Heckl, W. M. Carboxylic Acids: Versatile Building Blocks and Mediators for Two-Dimensional Supramolecular SelfAssembly. Langmuir 2009, 25, 11307−11321. (3) Xu, L.; Yang, L.; Lei, S. Self-assembly of conjugated oligomers and polymers at the interface: structure and properties. Nanoscale 2012, 4, 4399−4415. (4) Xu, H.; Wolffs, M.; Tomovic, Z.; Meijer, E. W.; Schenning, A. P. H. J.; De Feyter, S. A multivalent hexapod having 24 stereogenic centers: chirality and conformational dynamics in homochiral and heterochiral systems. CrystEngComm 2011, 13, 5584−5590. (5) Rabe, J. P.; Buchholz, S. Direct Observation of Molecular Structure and Dynamics at the Interface Between a Solid Wall and an Organic Solution by Scanning Tunneling Microscopy. Phys. Rev. Lett. 1991, 66, 2096−2099. (6) Stabel, A.; Heinz, R.; Rabe, J. P.; Wegner, G.; De Schryver, F. C.; Corens, D.; Dehaen, W.; Süling, C. STM Investigation of 2D Crystals of Anthrone Derivatives on Graphite: Analysis of Molecular Structure and Dynamics. J. Phys. Chem. 1995, 99, 8690−8697. (7) Ghijsens, E.; Cao, H.; Noguchi, A.; Ivasenko, O.; Fang, Y.; Tahara, K.; Tobe, Y.; De Feyter, S. Towards Enantioselective Adsorption in Surface-Confined Nanoporous System. Chem. Commun. 2015, 51, 4766−4769. (8) Hla, S.-W.; Bartels, L.; Meyer, G.; Rieder, K.-H. Inducing All Steps of a Chemical Reaction with the Scanning Tunneling Microscope Tip: Towards Single Molecule Engineering. Phys. Rev. Lett. 2000, 85, 2777−2780. (9) Alemani, M.; Peters, M. V.; Hecht, S.; Rieder, K.-H.; Moresco, F.; Grill, L. Electric Field-Induced Isomerization of Azobenzene by STM. J. Am. Chem. Soc. 2006, 128, 14446−14447. (10) Lapitan, L. D. S.; Tongol, B. J. V.; Yau, S.-L. Molecular Assembly and Electropolymerization of 3,4-Ethylenedioxy-thiophene on Au(111) Single Crystal Electrode as Probed by In Situ Electrochemical STM in 0.10 M HClO4. Langmuir 2010, 26, 10771−10777. (11) Gifford, L. K.; Sendroiu, I. E.; Corn, R. M.; Luptak, A. Attomole Detection of Mesophilic DNA Polymerase Products by NanoparticleEnhanced Surface Plasmon Resonance Imaging on Glassified Gold Surfaces. J. Am. Chem. Soc. 2010, 132, 9265−9267. (12) Shen, Z.; Huang, M.; Xiao, C.; Zhang, Y.; Zeng, X.; Wang, P. G. Nonlabeled Quartz Crystal Microbalance Biosensor for Bacterial Detection Using Carbohydrate and Lectin Recognitions. Anal. Chem. 2007, 79, 2312−2319. (13) Cheng, L.; Niu, L.; Gong, J.; Dong, S. Electrochemical Growth and Characterization of Polyoxometalate-Containing Monolayers and Multilayers on Alkanethiol Monolayers Self-Assembled on Gold Electrodes. Chem. Mater. 1999, 11, 1465−75. (14) Collman, J. P.; Devaraj, N. K.; Eberspacher, T. P. A.; Chidsey, C. E. D. Mixed Azide-Terminated Monolayers: A Platform for Modifying Electrode Surfaces. Langmuir 2006, 22, 2457−2464. (15) Li, X.; Wang, X.; Bai, Y.; Liu, H. Direct analysis in real time mass spectrometry: a powerful tool for fast analysis. Mass Spectrom. Lett. 2015, 6, 1−6. (16) Manova, R. K.; Joshi, S.; Debrassi, A.; Bhairamadgi, N. S.; Roeven, E.; Gagnon, J.; Tahir, M. N.; Claassen, F. W.; Scheres, L. M. W.; et al. Ambient surface analysis of organic monolayers using direct analysis in real time Orbitrap mass spectrometry. Anal. Chem. 2014, 86, 2403−2411. (17) Sen, R.; Escorihuela, J.; Smulders, M. M. J.; Zuilhof, H. Use of ambient ionization high resolution mass spectrometry for the kinetic analysis of organic surface reactions. Langmuir 2016, 32, 3412−3419. (18) Kang, H.; Kim, Y.; Choi, I.; Chang, R.; Yeo, W. S. Determination of self-exchange rate of alkanethiolates in selfassembled monolayers on gold using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. Anal. Chim. Acta 2014, 843, 38−45. 466
DOI: 10.1021/acs.langmuir.6b03252 Langmuir 2017, 33, 459−467
Article
Langmuir (39) Sakakura, A.; Kawajiri, K.; Ohkubo, T.; Kosugi, Y.; Ishihara, K. Widely useful DMAP-catalyzed esterification under auxiliary base- and solvent free conditions. J. Am. Chem. Soc. 2007, 129, 14775−14779. (40) Bhattarai, A.; Mazur, U.; Hipps, K. W. Desorption kinetics and activation energy for cobalt octaethylporphyrin from graphite at the phenyloctane solution - graphite interface: an STM study. J. Phys. Chem. C 2015, 119, 9386−9394. (41) Yang, Y.; Zimmt, M. B. Shape amphiphiles in 2D: Assembly of 1-D stripes and control of their surface density. J. Phys. Chem. B 2015, 119, 7740−7748. (42) Wu, Q.; Pomerantz, A. E.; Mullins, O. C.; Zare, R. N. Minimization of fragmentation and aggregation by laser desorption laser ionization mass spectrometry. J. Am. Soc. Mass Spectrom. 2013, 24, 1116−1122. (43) Iura, T.; Ohtani, H. Fragmentation behavior of poly(methylmethacrylate) during matrix-assisted laser desorption/ionization. Rapid Commun. Mass Spectrom. 2015, 29, 155−162. (44) Mali, K. S.; Adisoejoso, J.; Ghijsens, E.; De Cat, I.; De Feyter, S. Exploring the complexity of supramolecular interactions for patterning at the liquid-solid interface. Acc. Chem. Res. 2012, 45, 1309−20. (45) Adisoejoso, J.; Tahara, K.; Okuhata, S.; Lei, S.; Tobe, Y.; De Feyter, S. Two-dimensional crystal engineering: A four-component architecture at a liquid−solid interface,. Angew. Chem., Int. Ed. 2009, 48, 7353−7357. (46) Xue, Y.; Zimmt, M. B. Patterned monolayer self-assembly programmed by side chain shape: Four component gratings,. J. Am. Chem. Soc. 2012, 134, 4513−16. (47) Yang, Y.; Zimmt, M. B. Shape-directed patterning and surface reaction of tetra-diacetylene monolayers: Formation of linear and twodimensional grid polydiacetylene alternating copolymers. Langmuir 2015, 31, 12408−12416.
467
DOI: 10.1021/acs.langmuir.6b03252 Langmuir 2017, 33, 459−467
