Damage cross sections and surface track dimensions of biomolecular surfaces bombarded by swift-heavy-ions

JQ23 __

Nuclear Instruments and Methodsin Physics ResearchB 107( 1996)28I-286

_-

Beam Interactions with Yaterials & Atoms

@ ELSEVIER

Damage cross sections and surface track dimensions of biomolecular surfaces bombarded by swift-heavy-ions J. Eriksson

*,

J. Kopniczky,

P. Demirev, R.M. Papal&o, G. Brinkrnalm, P. Hhnsson, B.U.R. Sundqvist

C.T. Reimann,

Division of Ion Physics, Department of Radiation Sciences, Uppsala University, Box 535. S-751 21 Uppsala. Sweden

Abstract

We measured damage cross sections of the peptide LHRH for various MeV ions with the same velocity and compared the data with simple hit theory calculations. We employed scanning force microscopy to measure dimensions of surface tracks in amino acid L-valine induced by individual incident MeV ions of various stopping powers but with the same velocity. Areas of surface tracks and damage cross sections showed linear dependences on electronic stopping power. The area of a surface track of L-valine was significantly larger than previously obtained damage cross sections of amino acids for similar incident ions.

1. Introduction The energy electronically deposited in insulating targets by incident swift heavy ions leads to various modifications of the target material. Around the ion impact point, a plastic deformation of an initially flat surface can be observed e.g. by scanning force microscopy (SFM). For some materials the deformation is in the form of a crater [ 1,2]; for other materials it appears as a bump [3]. The former is indirect evidence of sputtering of the surface. It has also been shown that at large depths some materials are modified around the incident ion path resulting in latent track formation [4-71. Since the ion-solid interaction is statistical in its nature, there is no guarantee that the material modification along the incident ion trajectory is continuous. But, when the deposited energy density is high, one can observe that tracks are continuous [7], still keeping in mind that bulk track dimensions can differ from surface track dimensions [3]. In many spuUering experimenfs the material modification by incident ions is observed via the detection of ejecta. However, there is another means of monitoring the material modification. The typical observation for a molecular target is that the yield of intact sputtered molecules or molecular ions decreases with increasing fluence. This drop of the signal can lx caused both by a change in the shape of the surface due to plastic deformation, and by

* Corresponding author.

structural and chemical changes of the material around and along the ion path. The signal disappearance resulting from these processes is usually referred to as “damage” and is characterised by a “damage cross section” [8,9]. Below, we present damage cross sections for a biomolecular target (LHRH, a peptide composed of 10 amino acids) based on collector measurements of relative total sputtering yields of the molecules as a function of fluence [ions/cm’] for various primary ion beams with the same velocity, but with different stopping powers. Also. SFM was employed to measure the dimensions of surface tracks (craters) formed by individual swift heavy ions impacting on crystalline amino acid L-valine. The scaling of crater volume with electronic stopping power (d E/d x) was recently reported to be similar (volume a (d E/d x>~, y > 3) to the d E/d x scaling of the total yield of LHRH molecules [lo]. Damage cross sections have been used to estimate the dimensions of the surface area eroded in an ion impact [11,12]. Here, we specifically compare observed surface track areas with damage cross sections. We also compare our results with predictions of theoretical models. Swift heavy ion induced sputtering of biomolecules has found applications in mass spectrometry [ 131. Induced defects on biomolecular surfaces can be important in emerging fields of nanotechnology and materials science. The physical mechanisms in electronic sputtering of biomolecules are still not completely understood, partially because most of the data are for sputtered ions, but theories are mostly for neutrals 1141.

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repeated, starting with a virgin film, for each of the four ion beams employed. Directly after the irradiations, without breaking the vacuum; the collector plate was turned to the analysis position. A 78.2 MeV ‘*‘I ion beam (area = 0.1 cm’) was employed for PDMS analysis of each collector portion. The rate of incident ions during analysis was typically 3000 s-l. In a previous demonstrator experiment the same procedures as described above were performed for a smaller peptide tri-leucine (m = 357 u), where 72 MeV ‘27I ions were employed for irradiation and analysis. In order to check the response of the collector as a function of deposition of sputtered material we irradiated several virgin target spots with 78.2 MeV *27I ions and used one and the same collector portion. After irradiating each target spot a PDMS analysis of the collector portion was performed. For the SFM study, amino acid L-valine (Sigma Chemical Company, St. Louis, USA) was dissolved in deionised water to near saturation. Droplets of the solution were allowed to evaporate on a silicon backing. Leafy crystals, with their {lOO] faces parallel to the silicon surface as preferential orientation, were formed during evaporation [181. The L-valine targets were irradiated with charge equilibrated ions: 160, 32S, 79Br, and ‘*‘I, with a velocity of 1.1 cm/ns and incidence angles of 45”, 67” and 79” with respect to the surface normal. The ion beams were scanned to ensure uniform bombardments [19]. Fluences were in the range 0.5-2 X 10’ cm-*. SFM studies were performed under ambient conditions with a NanoScope@ 111(Digital Instruments) SFM operated in the TappingModeTM (TM)

T=pieavy

Faraday cup

Ions

4.3nA

Startdetector

Fig. 1. Collector experimental set-up: Upper part is set up for irradiation and collection. Lower part shows the collector plate turned to face an analysis beam and a time-of-flight mass spectrometer.

2. Experimental Human peptide LHRH (m = 1182 u, Sigma Chemical Company, St. Louis, USA) was dissolved in trifluoroacetic

acid and acetic acid (1: 4). The solution was electrosprayed onto an aluminised mylar backing to form a thick target (> 2 km>. An n-doped silicon plate of dimensions 1 X 7 cm* was employed as a collector. The collector plate was cleaned according to a procedure resulting in a hydrophilic surface [ 10,151. The target and the collector were mounted in a turbomolecular pumped chamber (pressure = 2 X 10B6 Torr). Both target and collector could be moved independently in the vertical direction. The collector could be turned either to face the target (irradiation position), or the grid of a time-of-flight mass spectrometer (analysis position, Fig. 1). A slit collimator (not shown) between the target and the collector in the irradiation position allowed sequential use of 10 X 5 mm* portions of the collector. In the analysis position heavy ions could impinge on the collector surface at a 45” angle of incidence. Plasma desorption mass spectrometry (PDMS) [13] spectra of different portions of the bare silicon collector plate were acquired in order to check the background contamination. Charge equilibrated ions [16,17]: 32S, 63Cu, 79Br, and ‘27I from the Uppsala EN-tandem accelerator were employed to irradiate the LHRH target at an incidence angle of 53 f 2” with respect to the surface normal. In each ion beam the particle velocity was 1.1 cm/ns and the beam current was typically 0.3 nA. One virgin LHRH target spot (area = 0.12 cm*) was irradiated sequentially with one type of ion. A fresh portion of the collector was employed in each step of the irradiation sequence. The procedure was

m-4.

3. Results 3.1. Collector experiment Mass spectra from each collector portion were analysed by computing the peak area [counts] of the (LHRH + H)+ ion peak (inset Fig. 2). The peak area, subtracted for background and divided by the number of start pulses, is referred to as the signal. Sticking probabilities are. important in absolute yield experiments employing collector plates or catcher foils [21]. We probed only the relative total yield of LI-IRH at various fluences and hence the value of the sticking probability is of limited importance. But, the various collector portions employed should have the same properties. Mass spectra of various bare silicon collector portions acquired before irradiation/collection showed no significant differences. The response of the collector to deposition of sputtered material from several virgin target spots is shown in Fig. 2. The signal was proportional to the amount of material deposited. Deviations from the fitted straight line probably

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16 iii E z -012

i i L

0

20

40

60

100

60

Fluence [IO” cm.‘1 /

0

2

4

6

I

8

10

I

I

12

14

16

Fluence [lO”ions/cm*]

Fig. 2. The response of tbe collector as a function of deposition of material sputtered from a target of LHRH by 78 MeV “‘I ions. ‘Ibe inset is a PDMS spectrum of a collector plate exposed to

sputteredLHRH molecules.

come mainly from yield variations over the target surface, and should be included in the error of a yield measurement. For the damage cross section this variation does not give any error since each cross section was determined from the drop of the yield from a particular target spot. In Fig. 3 we have plotted the cumulative signals as a function of the cumulative primary ion fluence (henceforth fluence) in the sequential irradiation of one LHRH target spot with 63Cu ions. It is seen that the signal drops off as a function of fluence. A function of the type F(4) = Y,,a- ‘( 1 - exp( - ~4)) was fitted to the data, where 4 is

Fig. 3. Cumulative collector signal as a function of fluence of 1.1 cm/ns 63Cu ions impinging one spot on tbe LHRH target. Tbe solid line is a fit to the function l’cl+- ‘(I- exp( - u@)l. Symbols am explained in the text.

fluence and (+ and Ye are fitting parameters. (+ is the damage cross section, F(4) is a measure of the relative yield integrated over fluence, and Ye is proportional to the yield of molecules sputtered from an undamaged target spot [22]. The same fitting procedure was performed for the four different primary ion beams. Measured damage cross sections are summarised in Table 1. Variations in the beam current and the uncertainty of the beam spot size were the main factors in the damage cross section error, estimated to be 20%. 3.2. SFM study Some details on the topography as well as widths and lengths of heavy ion induced surface tracks in L-valine have been presented elsewhere [2]. Here we concentrate on

Table 1 Measured and calculated damage cross sections in various bioorganic targets. The calculations were based on a density of 1.26 g/cm3 all materials and m = 1 in Eq. (2) dE/dx values were calculated using Bragg’s law and Ref. [23] Molecule

Mass

Ion

Iul

dE/dx

Measured

Calculated

yMeV/u]

N/Al

(T[nm*]

B [nm2]

valine + H a valine-H a

118

127I

0.71

900

685

18

60

116

127

I

0.71

900

44*

15

60

leucine b

131

127 I

357 1182 1182 1182 1182 5734

127 I

0.62 0.57 0.62 0.62 0.62 0.62 0.71

831 717 809 621 546 322 709

6Ort 27 l44+ 29 190+ 38 166* 32 102 f 20 88k 17 500 + 170

58 89 194 156 140 90 425

tri-leucine ’ LHRH ’ LHRH ’ LHRH ’ LHRH ’ insulin + H a

127 I

79Br 63cu 32s 127 r I

for

a Measured (T from Ref. [8]. b Measured (T from Ref. 1221. ’ Present work.

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the measured areas which are shown in Fig. 4 as a function of stopping power for 45”, 67“ and 79” incidence angles. The fitted power functions are characterised by a power of 1, 1.4 and 1, for 45”, 67” and 79” respectively.

4. Discussion As shown in Fig. 4 and Table 1 respectively, the dimensions of imaged surface tracks and measured damage cross sections are very different. A typical surface track in L-valine bombarded with 78 MeV ‘*‘I ions has an area of 700 nm* (67”) whereas measured damage cross sections of amino acids are around 60 nm* (60“). Simple model predictions of surface track dimensions [24-271 feature no increase in track areas with increasing molecular mass. Damage cross sections display a clear mass dependence (Table 1) and therefore also from the view of models differ from the behaviour of surface tracks. Volumes of craters in L-valine and total yields of leucine and LHRH molecules show similar steep dependences on d E/dx 110,221, in fair agreement with predictions of pressure pulse and shock wave models [25-271 where the yield scales as (d E/d x>~. The same models predict that the area of surface tracks scales as (d E/d x)*. Here our experiment on L-valine failed to reproduce the model prediction, since the observed d E/d x dependence was linear to slightly superlinear. As pointed out in Ref. [lo], valine is a relatively volatile biomolecule and therefore thermal models should not be ruled out from the discussion. The thermal spike model [24] predicts a linear dE/dx dependence of the surface track area, but a quadratic dependence for the yield. Thermal spike and other models involving the diffusion of energy demand detailed knowledge on material proper-

1000

I

10 ’ 10

100 ; g

(MeVom2.mg”)

Fig. 4. Surface track areas on L-valine as a function of electronic stopping power for various angles of incidence. Fitted power laws are character&d by powers of 1 to 1.4. The inset is two craters from single lz71 ion impacts on the L-valine surface.

ties in order to allow direct calculations of track dimensions or damage cross sections. Multiple hit theories based on Poisson statistics provide a more simplified approach to ion track phenomena [28,29]. Secondary electrons (b-particles) are produced in the highly excited region around the ion path and move outside that region. The stopping power of &particles in a solid of density p is approximately a constant: ~a = p/O.991 eV/A, for electron energies < 1 keV [30]. Assuming electron paths radially from the ion path, the radial range of the most energetic electrons is r= (3.9 X 105/ p) (u/c)* A, where v is the ion velocity and c is the speed of light [30]. The resulting energy density at a distance r from the ion path is e(r). We assume that a molecule with area A, is damaged if it is hit at least m times in an ion impact, and that the probability for x hits is Poisson distributed. Then the probability of at least m hits is: P(hits B m)

The damage cross section is obtained by integrating Eq. (1) over the range of the &particles: U= [r2nrP(hits

2 m) dr.

'0

We choose the molecular area A, =L*, where L = is the length of the molecule and M is the ( p/MY3 mass. We employ e(r) = (dE/dx)/4nr* In r eV/A3 [28], although more accurate expressions exist [31]. The average energy density in a molecule differs significantly from e(r) only if r < L [28]. Since L -=zr in our experiments, we neglected the variations in e(r) within a molecule. e(r) is not defined at r Q 1 A and therefore, for simplicity, Eq. (2) is calculated for 1 < r G T A,. In Fig. 5 it is seen that m = 1 gives the best agreement with the experiment, corresponding to a linear dE/dx scaling of the damage cross section. It is important to note that in our experiment, the damage cross section includes all processes that can lead to signal disappearance, e.g. diffusion of radicals and ion induced changes of the binding energy. Mechanical or electrical properties of the solid could change and thereby alter the properties of energy tmnsport in such a way that the sputtering probability is reduced. Also, a chemical modification of the target is indicated by the observation that the initially white target surface became brownish after irradiation, implying a degree of carbonisation. The density of carbon atoms is about the same in the biomolecules of Table 1. The damage cross section of C, measured by electronic sputtering [9] is about twice as high as would be predicted by our simple hit calculation (Eq. (2)). This discrepancy could indicate that the density

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ment with measured damage cross sections. Areas of surface tracks were one order of magnitude larger than damage cross sections of amino acids, which indicates that damage cross sections in sputtering experiments should be used with care for predicting surface track dimensions. Although crater volumes in L-valine and total yields of intact LHRH molecules show fair agreement with pressure pulse- or shock wave model predictions of the dependence on stopping power, measured crater areas failed to reproduce the predictions of these models.

Acknowledgements

0

10

20

30

40

50

60

70

80

l/p dOdx [MeVcm*/mg]

Fig. 5. Measured damage cross sections (points) compared with hit theory calculations (lines) for m = 1 and m = 2 in Eq. (2).

of carbon atoms plays a role in processes of signal disappearance, since the density of carbon atoms is higher for C, than for biomolecules. The processes of damaging or signal disappearance in sputtering experiments are not clear in detail. However, a simple hit theory approach gave good quantitative agreement with measured data, independently of whether measurements were based on total yields (LHRH, tri-leucine and leucine) or on the molecular ion yields (valine and insulin). Models typically do not consider the structure of the target material. This is a limitation in comparisons with experiments in cases where the target structure is well known. Also, differences between target structures in different experiments should be considered. Locally flat Lvaline surfaces may exhibit different sputtering properties than rough electrosprayed surfaces. Some structural aspects of layered materials (Langmuir-Blodgett films and L-valine) in sputtering experiments have been discussed recently 132,331. The basic idea in that discussion is that clustering may be an important factor for the width of a surface track and that the clustering is related to the material structure. Further experimental and theoretical investigations of sputtering of organic targets should include examinations of target structure dependences as well as considerations of effects of clustering on the observed ejecta and the resulting surface defects.

5. Conclusions Damage cross sections of LHRH and L-valine surface track areas showed approximately linear dependences on dE/dx. Simple hit theory calculations showed fair agree-

We thank A. Hallen and N. Keskitalo for their assistance in the irradiation of L-valine samples and the Swedish Natural Sciences Research Council (NFR), the Swedish Technical Research Council (TFR), and the Knut and Alice Wallenberg Foundation for financial support.

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