Chemical analysis of complex surface-adsorbed molecules and their

Subscriber access provided by MT ROYAL COLLEGE

Article

Chemical analysis of complex surface-adsorbed molecules and their reactions by means of cluster-induced desorption/ionization mass spectrometry Andre Portz, Markus Baur, Gordon Rinke, Sabine Abb, Stephan Rauschenbach, Klaus Kern, and Michael Dürr Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04876 • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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.

Analytical Chemistry 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 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

Analytical Chemistry

Chemical analysis of complex surface-adsorbed molecules and their reactions by means of cluster-induced desorption/ionization mass spectrometry Andre Portz,† Markus Baur,† Gordon Rinke,‡ Sabine Abb,‡ Stephan Rauschenbach,‡,¶ Klaus Kern,‡,§ and Michael Dürr∗,† Institut f¨ ur Angewandte Physik, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany, and Max Planck Institute for Solid State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany E-mail: [email protected]

January 23, 2018

∗

To whom correspondence should be addressed Institut f¨ ur Angewandte Physik, Justus-Liebig-Universität Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany ‡ Max Planck Institute for Solid State Research, Heisenbergstr. 1, D-70569 Stuttgart, Germany ¶ Present Address: Department of Chemistry, University of Oxford, Mansfield Road, Oxford, OX1 3TA, United Kingdom § ´ Institut de Physique, Ecole Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland †

1

ACS Paragon Plus Environment

Analytical Chemistry 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

Abstract Desorption/Ionization induced by Neutral SO2 Clusters (DINeC) is used for mass spectrometry (MS) of surface-adsorbed molecules. The method is shown to be a surface sensitive analysis tool capable to detect molecular adsorbates in a wide range of molecular weight as well as their reactions on surfaces, which are otherwise difficult to be accessed. Two different surface/adsorbate systems prepared by means of electrospray ion beam deposition were investigated: For the peptide angiotensin II on gold, intact molecules were desorbed from the surface when deposited by soft landing electrospray ion beam deposition (ES-IBD). By comparison to the well-controlled amount of substance deposited by ES-IBD, the sensitivity of DINeC-MS was shown to be in the order of 0.1 % of a monolayer coverage, corresponding to femtomol of analyte. Depending on deposition and sample conditions, the original state of charge of the molecules could be retrieved. Reaction of the adsorbed molecules both with surface atoms as well as with co-adsorbed D2 O was monitored. Rhodamine 6G was also desorbed as intact molecule when deposited with kinetic energies below 50 eV. For higher deposition energy, fragmentation of the dye molecules was observed by means of DINeC-MS.

2


Page 2 of 23

Page 3 of 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


INTRODUCTION The bottom-up approach to develop nano-structured materials, i.e., the formation of structures through self-assembly or self-organized growth, receives great attention due to the ability to generate complex supermolecular structures from relatively simple components. 1 In particular, the fabrication of biological interfaces, for instance by adsorption or attachment of biomolecules on various substrates, is intensively studied both with respect to fundamental questions on substrate-adsorbate interactions as well as with respect to potential applications. 2–5 The design of such systems will crucially depend on the complete understanding of their structure and chemical composition at the atomic level. The same holds for on-surface synthesis of covalently bound two-dimensional polymers from smaller molecular building blocks. 6–9 A wide range of surface sensitive probes such as photoelectrons, ion beams, or atomically sharp tips is employed in different surface analysis methods aimed at detecting the chemical composition and molecular structure of surface adsorbed nanostructures. 10 However, due to the complexity of both the molecular structure and the interaction with the surface, the investigation of larger molecules and their reactions on surfaces presents a particular challenge: the chemical information obtained by conventional surface analytical methods is often limited to information on the atomic species present but does not reveal their complex molecular arrangement. Among the surface analysis techniques that provide chemical information, secondary ion mass spectrometry (SIMS) is widely used for the investigation of surface adsorbates. SIMS provides a high sensitivity and is capable to detect small molecules or their fragments, from which the chemical information of the surface-adsorbed entity can be extracted. 11 The major disadvantage, however, is the high degree of fragmentation due to radiation damage caused by ion bombardment with the energetic primary ion beam, which can destroy valuable information. While fragmentation has been significantly reduced when charged molecular clusters were introduced as primary ions, 12–14 it still dominates SIMS spectra of 3



large macromolecules and biological samples. 15 Recently, desorption/ionization induced by neutral clusters (DINeC) was shown to be a soft and matrix-free ionization method for the mass spectrometric analysis of biomolecules. 16–20 DINeC employs supersonic beams of molecular clusters of 103 to 104 SO2 molecules directed at the sample surface. Their impact provides the energy necessary for desorption and, due to the high dipole moment of SO2 (1.6 Debye), they also serve as a transient matrix, dissolving the analyte molecule in the course of desorption. 21 As the latter process reduces the effective desorption barrier, low cluster energies of less than 1 eV/molecule can be employed. In combination with the rapid redistribution of the system’s energy after shattering of the clusters during and after surface impact, fragmentation of the desorbing molecules is largely suppressed. 16,19 Here we demonstrate that DINeC provides chemical information about complex, surfaceadsorbed molecules. To this end, well-defined model systems were fabricated with great precision by means of electrospray ion beam deposition (ES-IBD). This gentle, chemically selective, and highly clean vacuum deposition method for large, nonvolatile molecules was initially developed to enable high-resolution scanning probe microscopy (SPM) imaging of individual molecules adsorbed at surfaces in ultrahigh vacuum. 22 In particular, ES-IBD allows for a precise, quantitative control of the coverage as well as to choose between deposition of only intact molecules (soft landing) or deliberately creating fragments by hyperthermal collisions with the surface. 23,24 In our experiments, the dye molecule rhodamine 6G (Rho6G) and the peptide angiotensin II (ATII) were deposited on various substrates by means of ES-IBD. DINeC analysis shows that these molecules can be recovered from the surface in positive and negative polarity even at very low surface coverage of 0.1% of a monolayer (equivalent to an amount of substance in the femtomol regime); a linear relationship between coverage and signal intensity was established over three orders of magnitude. Intact molecules and fragmented adsorbates were clearly distinguished, as well as different charge states were detected depending on the

4


Page 4 of 23

Page 5 of 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


conditions of the deposition. The chemical interactions of the adsorbates with the substrate and coadsorbates were also observed as they lead to characteristic signatures in the DINeC mass spectrum. In the following, we show that these capabilities make DINeC especially useful for the analysis of complex molecular adsorbates and their reactions.

EXPERIMENTAL Samples were prepared using soft landing electrospray ion beam deposition (ES-IBD), 22,24 which makes use of intact molecular gas phase ions produced by means of electrospray ionization (ESI). 25 The ions were generated in ambient conditions and transported to the sample in ultra-high vacuum (UHV) through a differential pumping system of six stages, guided by radio frequency and dc-ion optics [Fig. 1(a)]. Mass selected ions can be deposited on the samples, a retarding grid energy detector was used to measure the kinetic energy of the beam, which allows the adjustment of the collision energy by biasing the substrate. The sample current was monitored to control the amount of substance deposited on the sample. Typical spray solutions consisted of 10−4 M analyte in an ethanol/water mixture. Some samples were investigated by means of scanning tunneling microscopy (STM) using a variable temperature STM (Omicron VT-STM) directly attached to the deposition chamber. Polycrystalline Au samples as well as Si wafers covered by their natural oxide were cleaned in acetone, ethanol, and finally ultrapure water prior to deposition. The polycrystalline gold samples and Si-wafer pieces were transferred to the DINeC apparatus through ambient environment. In addition, Au(111) surfaces of single crystals were prepared in ultrahigh vacuum (UHV) by repeated Ar-sputter (1 kV, 10 µA, 15 min) and annealing cycles (900 K, 10 min) and were coated in-situ by ES-IBD. On these samples, large, atomically flat Au(111) terraces with the characteristic herringbone-reconstruction were observed by means of STM (Fig. 1b). They were transported to the DINeC apparatus without breaking the vacuum using a transportable UHV container at a pressure well below 10−9 mbar.

5



Figure 1: (a) Scheme of the electrospray ion beam deposition experiment. (b) STM image of a submonolayer coverage (approx. 1%) of angiotensin II on Au(111) deposited by means of ES-IBD (approx. 10−13 mol per 0.2 cm2 ). Inset: Magnified view of a single ATII molecule with disk-like shape indicating adsorption as molecular rotor. (c) TOF-MS of the electrospray ion beam of ATII before m/z filtering (positive ions). (d) TOF-MS of the m/z-filtered ion beam only showing the doubly charged ATII ion [M+2H]2+ . For the DINeC measurements, an SO2 cluster beam was generated via supersonic expansion from a pulsed nozzle (repetition rate f = 2 Hz, effective opening time tpulse ≈ 500 µs, nominal orifice diameter 0.5 mm). With a gas mixture of about 3% SO2 in helium and the pressure applied at the nozzle of pnozzle ≈ 15 bar, a log-normal cluster size distribution with a mean cluster size between 103 and 104 molecules and a beam velocity ≤ 1.6 km/s was obtained. 26 The ions produced during cluster-surface impact were transferred through a biased grid (Vgrid = −30 V with respect to the sample), a dual funnel inlet (fif,1 = 0.8 MHz, fif,2 = 1.3 MHz and Vpp,1 = 353 V, Vpp,2 = 507 V chosen not to induce fragmentation of the biomolecules) and several octopolar ion guides into a modified commercial Paul-trap-based 6


Page 6 of 23

Page 7 of 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


mass spectrometer (amaZon speed, Bruker Daltonics, Bremen, Germany) for subsequent analysis 19 [Fig. 2(a)]. The mean beam diameter on the sample was ≈ 4 to 5 mm (FWHM of the Gaussian beam profile), similar to the size of the deposition spot of the ES-IBD samples.

RESULTS and DISCUSSION Intact Deposition and Desorption The mass spectra of the ATII ion beam, before and after mass selection of the doubly charged ions, are shown in Figs. 1(c) and (d), respectively. The spectra indicate a clean ion beam of intact, protonated ATII cations, of which the doubly charged species is selected for deposition. In Fig. 1(b), an STM image as observed during cool-down of a Au(111) surface with ATII molecules deposited by means of ES-IBD is shown. Single, well-separated ATII molecules are observed at the elbow sites of the Au(111) herringbone reconstruction. Due to their rotational motion, which is much faster than the STM imaging, most of them appear as a disc in the images. 27 At low temperature (40 K), fully immobilized highly ordered molecular networks of ATII appear. 4 DINeC mass spectra are shown in Figs. 2(b) and (c) for Au samples on which ATII was deposited either by means of ES-IBD [1000 pAh (= pico Ampere hours), with ambient sample transfer] or by drop-casting a µm-thick film. For both types of preparation, a main peak at m/z 1046.5 (1044.5) is observed in the positive (negative) ion spectrum indicating single charged ions of the form [M+H]+ and [M-H]− , respectively. The cation signal is one order of magnitude stronger than the anion signal for both samples in agreement with previous results. 17 In addition, minor peaks associated with the doubly charged ion [M+2H]2+ and singly charged dimers [2M+H]+ are observed. No beam induced fragment ions are observed within the limits of the signal-to-noise ratio (S/N > 103 ), neither in the spectra from the drop-cast nor in the spectra from the ES-IBD sample, in both, positive and in negative ion mode. This is a clear indication for soft deposition by means of ES-IBD as well as 7



DINeC’s capability to desorb intact molecules. We do not find a major difference when SiOx substrates are used for control experiments. Furthermore, almost identical spectra, also with respect to the relative intensities of the [M+2H]2+ , [M+H]+ , and [M-H]− peaks, are observed by DINeC from samples onto which ATII had been deposited by means of ES-IBD as singly charged negative ions. We thus conclude that the deposited ions lose their initial charge

Figure 2: (a) Sketch of the DINeC-MS set-up. (b) DINeC mass spectra obtained from a Au surface on which ATII molecules were deposited by means of ES-IBD (1000 pAh [ATII+2H]2+ deposited). Top: cations, bottom: anions, note the different scale for intensity. (c) DINeC mass spectra of ATII from a drop-cast film on a gold surface. Top: cations, bottom: anions, note the different scale.

8


Page 8 of 23

Page 9 of 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


state when the samples are transferred through the ambient environment after deposition. During this transfer, water molecules adsorb on the surface, which are likely to interact with the deposited molecular ions.

Concentration Dependence The preparation of surfaces via ES-IBD allows for a precise control of the coverage through integration of the deposited ion current at a known charge state. When doubly charged ATII ions are employed, an accumulated charge of 1000 pAh amounts to 2 × 10−11 mol; at a spot size of 0.2 cm2 , this corresponds to the deposition of a full monolayer (ML) coverage. The coverage depicted in Fig. 1(b) then corresponds to approx. 1 % of a monolayer. Note that it appears higher since many of the individual molecules are rotating and are thus imaged with a much increased surface area [inset Fig. 1(b)].

Figure 3: Intensity of the (M+H)+ signal (major peak) in DINeC-MS spectra of ATII on Au as a function of deposited amount of material as derived from the ion current integration in the ES-IBD experiment. Two independent sets of samples are shown (open and filled symbols). The signal has been referenced against a µm-thick drop-cast film of ATII, 17 measured before and after the measurement of the ES-IBD samples, to compensate for variations in the cluster beam intensity. Insets: DINeC-MS from samples with 2 × 10−11 mol (top) and 2 × 10−13 mol ATII (bottom).

9



The control of deposited coverage possible with ES-IBD is used in the following to gauge the sensitivity of DINeC-MS using samples of defined coverage, all transferred through ambient conditions. In Fig. 3, the DINeC signal intensity of the [M+H]+ signal from an ATII/Au sample is plotted as a function of the amount of substance of ATII that had been deposited by means of ES-IBD. A linear dependence of signal intensity on the amount of substance is observed over three orders of magnitude. Moreover, repeating the experiment led to the same values when the obtained intensity was calibrated by the signal of a reference sample consisting of a thick drop-cast ATII film; the latter is known to give a constant signal and thus allows for correcting for fluctuations of the SO2 beam intensity (the effective cluster beam intensity might vary by up to 20 % from day to day in the current set-up). While the fraction of desorbed and ionized molecules may vary significantly between different molecules and substrates, depending on their chemical nature and mutual interaction, the presented data give a good estimate for the detection limit of DINeC-MS, which is in the femtomol region (with S/N > 3). Taking into account the sample size of 0.2 cm2 , a surface coverage as low as 0.1 % of a monolayer was detected.

Charge Retention When the sample transfer is realized via a transportable UHV chamber, i.e., without breaking the vacuum, we observe a dependence on the original charge state of the deposited molecules, in contrast to the experiments with sample preparation in and transfer through ambient conditions. In Fig. 4, two spectra from Au(111) samples, which were prepared in UHV and on which ATII ions were deposited either in the 2+ or 1+ state are shown [Figs. 4(a) and (b), respectively]. The DINeC cationic mass spectra clearly exhibit a dominant peak for the doubly charged ions when the molecules were deposited in the 2+ state and a dominant singly charged peak for the molecules deposited in the 1+ state. This observation reveals that the molecules, to some extent, retain their original charge state even when they are in direct contact with the gold surface, as well as throughout the 10


Page 10 of 23

Page 11 of 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


Figure 4: (Color online) DINeC mass spectra of ATII deposited on Au(111) substrates by means of ES-IBD (approx. 0.3 ML). Deposition and sample transfer took place in ultrahigh vacuum (p < 10−9 mbar). (a) ATII deposited as [M+2H]2+ . (b) ATII deposited as [M+H]+ . Arrows indicate peaks associated with chemical reactions of the ATII molecule (see main text). Inset: magnification of the region around m/z = 1127. Red line: data; black line: simulation of the isotopic pattern for [ATII-Asp+Au]+ indicating that the Au+ ion carries the charge. desorption process. Close to the metal surface, the presence of the positive charge of the molecular ions can be compensated by the electronic charge density of the substrate (leading to the so-called image potential 28 ). The adsorbed molecular ions should thus be considered as a neutral molecule-substrate complex with one or two additional protons attached to a basic residue. 29,30 Due to the presence of the protons, desorption/ionization is more likely to produce higher charged ions. This observation agrees well with the charge retention observed in other molecular desorption studies. 31 In contrast, samples transferred through ambient conditions lose their initial state of charge through reaction with components of the air, in particular water molecules. Desorption/ionization is then similar to the process on drop-cast samples 21 and singly charged cations are predominant (Fig. 2).

11



Page 12 of 23

Detection of Chemical Reactions While structure, conformation, and many other properties of molecules on surfaces can be measured by scanning probe microscopy with great precision even for a single molecule, obtaining chemical information is a great challenge.

Especially for complex functional

molecules, a chemical characterization of the molecular adsorbates on surfaces is required for a meaningful interpretation of the data obtained from these samples by other methods. 22,32,33 Such chemical information is obtained by means of DINeC-MS as first illustrated in Figs. 4(a) and (b), where, in addition to the main peaks associated with the intact ATII molecules, peaks are observed at m/z 1000.6, 1028.5, and 1126.5. The first two of these peaks are attributed to ATII molecules having lost a carboxylic acid or water group, respectively. The latter peak is attributed to ATII molecules without the terminal aspartic acid group but with an additional Au atom as adduct, [M-Asp+Au]+ ; a similar signature was observed when analyzing soft-landed Au clusters on self assembled monolayers. 34 Since these peaks are not found on the ex-situ prepared samples after ambient transfer (compare cation signal in Fig. 2), apparently a part of the ATII ion-surface complexes underwent a reaction on the in-situ prepared Au surface. Such surface reactions of the ATII molecules can be followed in real-time by means of DINeC-MS as illustrated in Fig. 5 for H/D isotope exchange. In Fig. 5(a), the isotope pattern of the singly charged, undeuterated ATII is shown as obtained by means of DINeCMS from a sample covered by ATII in the sub-monolayer regime [ES-IBD, 100 pAh ATII on Au(111)]. Exposing this sample to a partial pressure of 10−5 mbar D2 O leads to a significant shift and broadening of the isotope pattern [Fig. 5(b)], indicating replacement of H by D atoms in the ATII molecule. Whereas the first H atoms are exchanged rather quickly, the process significantly slows down towards higher degrees of deuteration [Fig. 5(c)], indicating different reaction rate constants being operative in the molecule, in agreement with detailed investigations of H/D exchange by means of DINeC-MS in bulk ATII samples. 35 In the case of ES-IBD, surface reactions, in particular fragmentation, of the adsorbates 12


Page 13 of 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


can be further controlled by means of the initial kinetic energy the molecules are deposited with. Fig. 6 shows DINeC-MS of Au samples on which Rho6G [Fig. 6(a),inset] was deposited at the same coverage (200 pAh) but with different initial kinetic energy ranging from 5 eV to 120 eV together with a reference spectrum obtained from a drop-cast sample. In addition to the main peak at m/z 443, one observes a dominant fragment peak at m/z 415, which is attributed to a McLafferty rearrangement including the loss of an ethylene entity. 19 Different to the drop cast reference sample, in the spectra from the ES-IBD samples this fragment peak is observed even at lowest energy, albeit with low intensity. This can be understood taking into account that the ES-IBD samples had been transported and stored at ambient conditions for several days prior to the measurement. When a drop cast reference sample was treated equivalently, the occurrence of the fragment peak was also detected. We thus

Figure 5: (Color online) Progressive change of DINeC mass spectra with increasing dose of D2 O. (a) Initial isotope pattern before dosing D2 O. (b) Mass spectrum after dosing D2 O for 100 s and (c) for 200 s at p = 1.4 × 10−5 mbar. Black lines are superpositions of isotope patterns including a range of degrees of deuteration d spanning from d = 3 to 11 in (b) and d = 6 to 12 in (c). 13



Figure 6: (a) DINeC mass spectrum of a µm-thick rhodamine 6G film prepared by means of drop casting. Inset: structure of Rho6G. (b) to (e) DINeC mass spectra (cations) from samples prepared by ES-IBD with different kinetic energy of the deposited ions as indicated. Total charge deposited was 200 pAh for each sample. Arrows indicate m/z = 415. attribute the intensity of the m/z 415 peak at 5 eV to degradation in the sample after deposition, the more as no fragmentation was observed for Rho6G under these conditions in previous and reference experiments. 24 When comparing the spectra for samples deposited at different kinetic energies, the absolute intensity decreases both for the peak associated with the intact molecules as well as for the fragment peak for a deposition at 40 eV, whereas it remains constant for the intensity of the intact molecule between 40 and 80 eV deposition energy, and decreases again for the highest deposition energy, 120 eV [Fig. 7(b)]. These measurements are consistent for different 14


Page 14 of 23

Page 15 of 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


Figure 7: (a) STM-image (U = −1.9 V, I = 14 pA, T = 40 K) of Rho6G on Au(111). The molecules adsorb preferentially at elbow sites of the herringbone reconstruction leading to the apparent ordered structure. (b) Initial intensity of the peaks at m/z = 443, i.e. the intact Rho6G molecule, and its main fragment at m/z = 415. Data of two different, independent data sets are shown. (c) Relative intensity of the peak at m/z = 415 with respect to the peak of the intact molecules at m/z = 443 subtracting a contribution to the m/z = 415 fragment which is associated with fragmentation on the sample during transfer through ambient (for details see main text). sample sets leading to the characteristic dependence of relative and absolute intensity on kinetic energy as displayed in Figs. 7(b) and (c). For the relative intensity of the fragment peak shown in Fig. 7(c), the fragment intensity was corrected for degradation using the 15



intensity of the 5-eV-spectrum as a baseline. It is then obvious that significant fragmentation is induced only at kinetic energies higher than 40 eV. A similar experiment with Rho6G cations deposited onto SiOx surfaces also showed an increase in fragment intensity above this value as observed by SIMS using a 25 keV Ga+ analysis beam. 24 As in the latter experiments no drop in the overall intensity was observed for energies below 100 eV, we attribute the decrease in signal intensity between 5 and 40 eV to changes in the chemical binding of the dye molecules to the surface. On clean Au(111) samples, the Rho6G molecules preferentially occupy elbow sites of the herringbone structure [Fig. 7(a)]. Such energetically more stable adsorption sites might not be accessible under soft-landing conditions on the ex-situ prepared samples. On the other hand, with increasing kinetic energy, the molecules may access preferential adsorption sites or even create pinning sites, 36 leading to stronger binding and thus less desorption in DINeC-MS, as only DINeC but not SIMS is sensitive on such changes of the binding to the surface. In the range between 40 and 80 eV, the total signal intensity then stays constant and the onset of fragmentation leads to an increase of both the absolute as well as the relative intensity of the fragment signal at m/z = 415. Above 100 eV, the total intensity again decreases for both peaks, but less pronounced for the fragment peak indicating further fragmentation at higher energies. A similar loss in total signal intensity at higher deposition energies was also observed in previous SIMS experiments and is attributed to the onset of additional processes such as scattering and/or desorption. Although the qualitative dependence of the relative signal intensity of the fragment peak is the same for the DINeC and SIMS mass spectra, we would like to note that quantitatively the intensity ratios differ for the two measurements; this might be attributed, e.g., to different relative ionization probabilities for the intact molecule and the fragment in the two ionization techniques.

16


Page 16 of 23

Page 17 of 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


CONCLUSION The application of DINeC-MS to samples prepared by means of ES-IBD demonstrates its capability for the analysis of surface-adsorbed complex molecules such as oligopeptides and dyes. DINeC combines several advantages: (1) It is a versatile, soft ionization method even for large molecules which can be applied without additional surface preparation. (2) It is extremely sensitive and inherits the high specificity from mass spectrometry. (3) The obtained information is quantitative. When compared with standard methods such as SIMS or MALDI, the latter also exhibits very soft desorption but requires sample preparation which is not compatible with an all-vacuum type experiment. On the other hand, SIMS does not require such sample preparation; however, even under mildest conditions (low energy primary cluster ions) significant fragmentation occurs in the case of biomolecules and the intensity of the fragment ion peaks is in the same order of magnitude as the peak associated with the intact biomolecule. 15 For DINeC, such beam induced fragments are not detected within the given S/N ratio. Furthermore, DINeC is quantitative, even for mixtures of different substances, 37 what is typically not the case for SIMS (so-called ”matrix effect“). 38 In particular, the combination of DINeC-MS with imaging on the molecular scale using SPM should be highlighted, because only combined chemical and structural information with high resolution is able to reveal the complexity as it can be reached in supermolecular architectures, e.g., built with novel fabrication methods such as ES-IBD. We showed that this goes well beyond confirming the integrity of the preparation. As an example, complex hyperthermal surface chemistry is possible with ES-IBD, which is not directly accessible even with high resolution SPM images but by means of DINeC-MS: intact ions were desorbed and analyzed in the case of sample preparation in the soft landing mode but reactions of the molecules induced by the kinetic energy during ion beam deposition were detected at higher deposition energies. The detection of surface reactions with co-adsorbates was further illustrated with the help of H/D isotope exchange in ATII, which was monitored in real-time. 17



In the field of supermolecular architectures on surfaces, DINeC-MS might thus contribute to the analysis of the chemical nature of intermediate and final products. In particular, we have seen that in-situ DINeC investigations of ES-IBD samples allows for an unprecedented precision in tracking the surface chemistry of large molecules which can serve as building blocks for such architectures. In more general, DINeC-MS was shown to be a very powerful methodology for the chemical characterization of surface-adsorbed complex molecules, such as biomolecules, and their reactions on surfaces.

Author information Corresponding author E-mail: [email protected]; phone +0049 (0)641 9933490 (M.D.). Notes The authors declare no competing financial interest.

18


Page 18 of 23

Page 19 of 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


References (1) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671–679. (2) Stutzmann, M.; Garrido, J. A.; Eickhoff, M.; Brandt, M. S. phys. stat. sol. 2006, 203, 3424 – 3437. (3) Adler-Abramovicha, L.; Gazit, E. Chem. Rev. 2014, 43, 6881 – 6893. (4) Abb, S.; Harnau, L.; Gutzler, R.; Rauschenbach, S.; Kern, K. Nat. Commun. 2016, 7, 10335. (5) Rauschenbach, S.; Rinke, G.; Gutzler, R.; Abb, S.; Albarghash, A.; Le, D.; Rahman, T. S.; D¨ urr, M.; Harnau, L.; Kern, K. ACS Nano 2017, 11, 2420 – 2427. (6) Grill, L.; Dyer, M.; Lafferentz, L.; Persson, M.; Peters, M.; Hecht, S. Nature 2007, 2, 687–691. (7) Lindner, R.; K¨ uhnle, A. Accounts of Chemical Research 2015, 16, 1582–1592. (8) Dong, L.; Liu, P. N.; Lin, N. Accounts of Chemical Research 2015, 48, 2765–2774. (9) Björk, J. Journal of Physics: Condensed Matter 2016, 28, 083002. (10) Vickerman, J. C.; Gilmore, I. Surface analysis – the principle techniques, 2nd Ed.; Jon Wiley & Sons: New York, 2009. (11) Vickerman, J. C.; Briggs, D. TOF-SIMS – materials analysis by mass spectrometry, 2nd Ed.; IM Publications: Chichester, 2013. (12) Winograd, N. Anal. Chem. 2005, 77, 143A – 149A. (13) Ichiki, K.; Ninomiya, S.; Nakata, Y.; Honda, Y.; Seki, T.; Aoki, T.; Matsuo, J. Appl. Surf. Sci. 2008, 255, 1148 – 1150.

19



Page 20 of 23

(14) Mochiji, K.; Hashinokuchiy, M.; Moritani, K.; Toyoda, N. Rapid Commun. Mass Spectrom. 2009, 23, 648 – 652. (15) Yokoyama, Y.; Aoyagi, S.; Fujii, M.; Matsuo, J.; Fletcher, J. S.; Lockyer, N. P.; Vickerman, J. C.; Passarelli, M. K.; Havelund, R.; Seah, M. P. Anal. Chem. 2016, 88, 3592 – 3597. (16) Gebhardt, C. R.;

Tomsic, A.;

Schröder, H.;

D¨ urr, M.;

Kompa, K.-L.

Angew. Chem. Int. Ed. 2009, 48, 4162 – 4165. (17) Baur, M.; Lee, B.-J.; Gebhardt, C. R.; D¨ urr, M. Appl. Phys. Lett. 2011, 99, 234103–1 – 234103–3. (18) Lee, B.-J.; Baur, M.; Gebhardt, C. R.; D¨ urr, M. Rapid Commun. Mass Spectrom. 2013, 27, 1090. (19) Baur, M.; Gebhardt, C. R.; D¨ urr, M. Rapid Commun. Mass Spectrom. 2014, 28, 290 – 296. (20) Portz, A.; Baur, M.; Gebhardt, C. R.; D¨ urr, M. Biointerphases 2016, 11, 02A316–1 – 02A316–5. (21) Portz, A.; Baur, M.; Gebhardt, C. R.; Frank, A. J.; Neuderth, P.; Eickhoff, M.; D¨ urr, M. J. Chem. Phys. 2017, 146, 134705–1 – 134705–5. (22) Rauschenbach, S.; Ternes, M.; Harnau, L.; Kern, K. Annual Review of Analytical Chemistry 2016, 9, 473 – 498. (23) Rauschenbach, S.; Stadler, F. L.; Lunedei, E.; Malinowski, N.; Koltsov, S.; Costantini, G.; Kern, K. Small 2006, 4, 540 – 547. (24) Rauschenbach, S.; Vogelgesang, R.; Malinowski, N.; Gerlach, J. W.; Benyoucef, M.; Costantini, G.; Deng, Z.; Thontasen, N.; Kern, K. ACS Nano 2009, 10, 2901 – 2910. 20


Page 21 of 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


(25) Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. Mass Spectrom. Rev. 1990, 9, 37 – 70. (26) Eusepi, F.; Tomsic, A.; Gebhardt, C. R. Anal. Chem. 2003, 75, 5124 – 5128. (27) Gimzewski, J. K.; Joachim, C.; Schlittler, R. R.; Langlais, V.; Tang, H.; Johannsen, I. Science 1998, 281, 531 – 533. (28) Lang, N. D.; Kohn, W. Phys. Rev. B 1973, 7, 3541 – 3550. (29) Vitali, L.; Levita, G.; Ohmann, R.; Comisso, A.; Vita, A. D.; Kern, K. Nature Materials 2010, 9, 320 – 323. (30) Pia, A. D.; Riello, M.; Floris, A.; Stassen, D.; Jones, T.; Bonifazi, D.; Vita, A. D.; Costantini, G. ACS Nano 2014, 8, 12356 – 12364. (31) Laskin, J.; Wang, P.; Hadjar, O.; Futrell, J. H.; Alvarez, J.; Cooks, R. G. International Journal of Mass Spectrometry 2007, 265, 237 – 243. (32) Zhang, Y.-H.; Kahle, S.; Herden, T.; Stroh, C.; Mayor, M.; Schlickum, U.; Ternes, M.; Wahl, P.; Kern, K. Nature communications 2013, 4, 2110. (33) Payer, D.; Rauschenbach, S.; Konuma, M.; Virojanadara, C.; Starke, U.; Dietrich, C.; Collin, J., J.P.and Sauvage; Lin, N.; Kern, K. J. Am. Chem. Soc. 2007, 129, 15662– 15667. (34) Johnson, G. E.; Laskin, J. International Journal of Mass Spectrometry 2015, 377, 205 – 213. (35) Portz, A.; Gebhardt, C. R.; D¨ urr, M. J. Phys. Chem. B 2017, 121, 11031 – 11036. (36) Bromann, K.; Brune, H.; Felix, C.; Harbich, W.; Monot, R.; Buttet, J.; Kern, K. Surface Science 1997, 377, 1051–1055. (37) Portz, A.; Aoyagi, S.; D¨ urr, M. Biointerphases 2018, (accepted). 21



(38) Shard, A. G.; Spencer, S. J.; Smith, S. A.; Havelund, R.; Gilmore, I. S. International Journal of Mass Spectrometry 2015, 377, 599 – 609.

22


Page 22 of 23

Page 23 of 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


Graphical abstract:

23


Chemical analysis of complex surface-adsorbed molecules and their

Recommend Documents