New Substrates for Polymer Cationization with Time-of-Flight

been studied with time-of-flight secondary ion mass spectrometry (ToF-SIMS). ... Fragmentation pattern dependence on polymer molecular weight and the ...
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Langmuir 2000, 16, 6503-6509

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New Substrates for Polymer Cationization with Time-of-Flight Secondary Ion Mass Spectrometry Roger Michel,† Reto Luginbu¨hl,† Daniel J. Graham,† and Buddy D. Ratner*,†,‡ Department of Bioengineering, University of Washington Engineered Biomaterials, and Department of Chemical Engineering, University of Washington, Seattle, Washington 98195-1720 Received November 2, 1999. In Final Form: February 21, 2000 Cationization of large polymers and their fragments on novel self-assembled monolayer substrates has been studied with time-of-flight secondary ion mass spectrometry (ToF-SIMS). Low (1 kDa) and high (400 kDa) molecular weight poly(ethylene oxide) (PEO) were spin-coated on metal ion substituted, carboxylic terminated, self-assembled monolayers on gold. Both cationized fragments and additional whole molecular species were observed for the 1 kDa PEO, while for 400 kDa PEO, only chain fragments of up to 30 monomer units were detected. Fragmentation pattern dependence on polymer molecular weight and the nature of the substrate is discussed. The intensity of the cationized fragments was found to depend on the metal ion used. Complementary experiments carried out on methyl terminated self-assembled monolayers suggested that metal ions that are desorbed from the gold surface, and therefore not in close proximity to the polymer, do not cationize polymers and their fragments. The substrates used in this work are simple to prepare, reproducible, and provide a model system for studies on the cationization of both polymers and proteins. Also, these ionically terminated self-assembled monolayer substrates open up new analytical possibilities for static SIMS.

Introduction Over the past decades, time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been used extensively to analyze polymers and their surfaces.1-4 ToF-SIMS allows for analysis of the outermost surface layers that are ejected after primary ion bombardment.5 It has been shown that the emitted ions are related to the chemical structure of the analyzed material and consist of molecular and quasimolecular ions, as well as other fragments from the surface. Because of their chemical structure, larger radicals and molecular ions (>100 Da) ejected from polymers are often in an electrically unstable state and are neutralized or fragmented during the desorption process. This makes it difficult to desorb and detect molecular ions and fragments from polymers that are of more than approximately 300 mass units. Enhanced detection of larger mass fragments can occur through cationization.6-10 Silver,7,11 lithium,12,13 †

Department of Bioengineering. Department of Chemical Engineering. * To whom correspondence should be addressed.

‡

(1) Wien, K. Nucl. Instrum. Methods Phys. Res., Sect. B 1997, 131, 38-54. (2) Benninghoven, A. Surf. Sci. 1994, 299-300, 246-60. (3) Eynde, X. V.; Bertrand, P.; Galuska, A. A. Surf. Interface Anal. 1997, 25, 878-888. (4) Leeson, A. M.; Alexander, M. R.; Short, R. D.; Briggs, D.; Hearn, M. J. Surf. Interface Anal. 1997, 25, 261-274. (5) Vickerman, J. C. Secondary Ion Mass Spectrometry; Oxford Science Publications: Oxford, 1989. (6) Zimmermann, P. A.; Hercules, D. M.; Benninghoven, A. Anal. Chem. 1993, 65, 983-991. (7) Grade, H.; Winograd, N.; Cooks, R. G. J. Am. Chem. Soc. 1977, 99, 7725-7726. (8) Xu, K.; Proctor, A.; Hercules, D. M. Int. J. Mass Spectrom. Ion Processes 1995, 143, 113-129. (9) Bletsos, I. V.; Hercules, D. M.; Greifendorf, D.; Benninghoven, A. Anal. Chem. 1985, 57, 2384. (10) Colton, R. J.; Murday, J. S.; Wyatt, J. R.; DeCorpo, J. J. Surf. Sci. 1979, 84, 235-248. (11) Moon, D. W.; Bleiler, R. J. J. Am. Chem. Soc. 1983, 105, 29162917. (12) Giessman, U. R.; F. W. Org. Mass Spectrom. 1976, 11, 10941100.

and sodium14,15 are some of the metal ions (X+) known to complex with sputtered molecules (M) to form cationized fragments (M + X)+. These larger ion complexes are more stable than their noncationized counterparts and are readily detected. Previously, Benninghoven and co-workers reported experiments with polymers and peptides on etched silver substrates.2,16,17 However, the cationization process at the substrate/polymer interface is not well understood, in part because of the complexity of the etched silver surface. Thus, further investigation of the cationization phenomenon requires a model system amenable to detailed characterization. To this end, we describe a new system for cationization of polymers. Our approach entails creation of ordered metal ion monolayers on well-characterized alkanethiol self-assembled monolayer surfaces. This approach allows for better control and manipulation of the cationization process during secondary ion formation. Self-assembled monolayers (SAMs) have been studied for almost two decades and can be used as model systems because of their well-defined orientation and molecular arrangement. The surface chemistry of the SAMs can easily be modified by introducing functional groups at the terminal (ω) position of the alkane chain. In the present work, a system based on carboxylic acid terminated SAMs was chosen as it allows for replacement of the acidic hydrogen by a metal ion suitable for the cationization process. Metal ions such as Na+, Li+, Ag+, Cu2+, Ba2+, and Tl+ were used for their ability to form metal ion/carboxylate complexes and their potential suitability for enhancing (13) Roellgen, F. W.; Borchers, F.; Giessmann, U.; Levsen, K. Org. Mass Spectrom. 1977, 12, 541-542. (14) Cox, X. B.; Linton, R. W.; Bursey, M. M. Int. J. Mass Spectrom. Ion Processes 1984, 55, 281-90. (15) Gusev, A. I.; Choi, B. K.; Hercules, D. M. J. Mass Spectrom. 1998, 33, 480-485. (16) Benninghoven, A.; Sichtermann, W. Heyden and Sons Ldt. 1977, 12, 595. (17) Benninghoven, A.; Sichtermann, W. K. Anal. Chem. 1978, 50, 1180-1184.

10.1021/la991436z CCC: $19.00 © 2000 American Chemical Society Published on Web 07/12/2000

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cationization. Electron spectroscopy for chemical analysis (ESCA) was used to monitor and optimize surface composition and metal ions exchange processes. The substrates were used for analysis of the ultrathin deposited polymer layers. Ideally, the ions at the interface between the monolayer and deposited polymer will break free and cationize large polymer fragments under SIMS analysis. These cation substrates open up new analytical possibilities for static SIMS and for understanding the cationization phenomena. We believe this is the first application of SAMs used for cationization and the first well-defined and characterized cationization substrate system. Materials and Methods Sample Preparations. All organic solvents were of HPLC grade (Aldrich), except ethanol (dehydrated, 200 Proof, McCormick), and were used without further purification. Silicon wafers diced into 10 mm × 10 mm pieces were cleaned in 2-propanol and methylene chloride. Subsequently, they were sputter-coated with a 20 nm chromium interlayer and 30 nm gold. The samples were either used immediately or stored up to 7 days under ethanol. Self-assembly of alkanethiols was carried out by immersing the samples for 48-72 h in 0.5 mM solutions of mercaptohexadecanoic acid (in-house synthesis) and hexadecylmercaptan (92% pure, Aldrich) in ethanol. Following the adsorption of the monolayer, they were rinsed with ethanol and then sonicated in ethanol for 2 min to remove physisorbed molecules. After renewed rinsing in ethanol they were dried with and stored under nitrogen. In a subsequent step, the acidic hydrogens of mercaptohexadecanoic acid were substituted with silver, sodium, lithium, barium, thallium, and copper ions. Silver ion substitution was carried out by dipping the prepared monolayer substrates in a 10 mM solution of AgNO3 (99.9999% pure, Aldrich) in acetone for 1 min. Substitution with sodium, lithium, and barium ions was achieved by immersing the samples in a 10 mM solution of the respective metal hydroxide (NaOH, VWR Scientific; LiOH, 99.995% pure, Aldrich; Ba(OH)2, 99.995% pure, Aldrich) solution in water for 1 min. Copper ion substitution was obtained by submersing the samples in a 20 mM solution of CuClO4 (98% pure, Aldrich) in ethanol for 2 min, whereas thallium ion substitution was achieved by immersing the substrates in a 10 mM solution of TlC2H5O (98% pure, Aldrich) in ethanol for 1 min. All samples were rinsed with solvent three times for 10 s immediately after exposure, then dried with nitrogen, and stored. Reference samples were prepared by etching a thin silver foil (0.25 mm, 99.9985% pure, Alfa Parabolic) for 3 min in nitric acid solution (20%), washing with water and ethanol, and drying with nitrogen. Deposition of the polymers onto the SAMs was done by spincoating (Headway Research EC101 spin-coater). Poly(ethylene oxide) (PEO) 1 kDa (MW 1000 g/mol, Aldrich) and PEO 400 kDa (MW 400 000 g/mol, Aldrich) were spin-cast onto the substrate surfaces. PEO(1 kDa) was diluted in methylene chloride at a concentration of 0.6 mg/mL. PEO(400 kDa) was diluted in methanol at a concentration of 0.5 mg/mL. One 5 µL drop was cast onto the sample spinning at 6000 rpm for 20 s. After polymer deposition, the samples were blown dry with nitrogen and stored until analysis. Control samples were prepared by immersing a CH3 terminated self-assembled monolayer in the metal ion containing solutions followed by deposition of the PEO using the protocols described above. ESCA Analysis. Analysis was performed on Surface Science Instruments (SSI) X-Probe and S-Probe ESCA instruments equipped with aluminum KR 1,2-monochromatized X-ray sources. The energy of the emitted electrons was measured with a hemispherical energy analyzer at pass energies of 50 eV (highresolution spectrum) and 150 eV (survey and detail spectra). SSI data analysis software was used to calculate the elemental compositions from the peak areas and to peak fit the highresolution spectrum. The binding energy (BE) scale was referenced by setting the CHx peak maximum in the C 1s spectrum

Michel et al. Table 1. Angular Dependent ESCA Results for Ag, Cu, Na, and Tl on Metal Ion Substituted COOH-SAMs at 0°, 55°, and 80° Takeoff Angles (deg)a (a) Metal ion Concentration takeoff angle

Ag 3d5/2 (%)

Cu 2p3/2 (%)

Na 1s (%)

Tl 4f7/2 (%)

0 55 80

5.2 5.4 5.0

1.7 2.0 1.6

2.6 2.7 2.2

3.3 3.8 4.6

(b) Metal Ion/Oxygen Ratio takeoff angle

Ag/O ratio

Cu/O ratio

Na/O ratio

Tl/O ratio

0 55 80

0.75 0.63 0.47

0.19 0.16 0.09

0.48 0.37 0.26

0.46 0.45 0.42

a Metal ion concentration (a) is shown as well as Me+/O ratio (b). Ideally, a full metal ion exchange (-COO-Me+) without adsorption onto the gold interface should equal Me+/O ) 0.5 at all takeoff angles.

to 285.0 eV.18 Peak counts that were no more than double the background noise were not quantified and, when corresponding to the binding energy of elements, were marked as “traces of ...”. Typical pressures in the analysis chamber during spectral acquisition were 10-9 Torr. Angle-dependent ESCA spectra were collected with the wide-angle acceptance lens masked with a 12° slit. Spectra were collected at 80°, 55°, and 0° takeoff angles, where the takeoff angle is defined as the angle between the surface normal and the analyzer lens. Time-of-Flight Secondary Ion Mass Spectrometry. TOFSIMS spectra were acquired using a model 7200 Physical Electronics PHI instrument (Eden Prairie, MN). The 8 keV Cs+ ion source was operated with a current of 1.5 pA, a pulse length of 10 µs, and a repetition rate of 3 kHz (0-2000 m/z spectra). The resulting ion dose was maintained below 2 × 1012 ions/cm2, on an area of 0.01 mm2. The secondary ions were extracted into a two-stage reflectron time-of-flight mass analyzer with a potential of 3 kV. A secondary ion-focusing lens between the analyzer entrance and drift region was held at 1 kV, promoting high angular acceptance and good transmission of ions. The bandpass of the analyzer was 100 eV, and an independent adjustable grid voltage (deceleration) allows energy focusing to be performed. The ions were postaccelerated to 10 kV and converted to charge pulses by a stacked pair of chevron-type multichannel plates (MCP). The signals were detected using a 256 stop time-to-digital converter (TDC) with 156 ps time resolution.

Results and Discussion ESCA Analysis. ESCA analysis of gold surfaces modified with self-assembled alkanethiol monolayers revealed a typical surface composition consisting of gold, carbon, sulfur, and oxygen. The survey spectra are in agreement with the literature.19 Angular dependent ESCA was performed on silver, copper, sodium, and thallium ion substituted SAMs and revealed that, for all four samples, the amount of gold and sulfur decreased while carbon and oxygen increased with increasing takeoff angle. This behavior is consistent with previously measured carboxylic terminated self-assembled monolayers. Analysis of the silver ion substituted samples revealed that the amount of silver did not vary as a function of takeoff angle (Table 1). Therefore, we conclude that, apart from the formation of the COO-Ag+ groups, silver also spontaneously adsorbs through the monolayer onto the (18) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Mulienberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1979. (19) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press Limited: New York, 1991.

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Figure 1. Negative ion ToF-SIMS spectrum of silver ion substituted (lower spectra) and unsubstituted (upper spectra) carboxylic terminated self-assembled monolayer. Both samples show the characteristic (M-H) from the monolayer at m/z 287.2 whereas only the substituted SAM shows peaks occurring from the monolayer molecule with the silver ion at m/z 393.1 and 395.1.

gold surface. This reaction readily occurs as silver exhibits a high affinity to gold since the gold surface provides electrons to reduce silver ions. The result is a doublelayered structure with silver present at the bottom and top of the SAM. This mechanism is confirmed by the change in Ag/O ratio at different takeoff angles (TOA) (Table 1). Oxygen-bearing groups are only present at the very top of the SAM. If the silver ions react only with the acidic hydrogen without adsorbing to the gold surface, they would exhibit a constant Ag/O ratio as a function of TOA. However, since silver also adsorbs to the interface, the ratio of Ag/O increases with decreasing TOA. The affinity and diffusion of metals such as silver and copper onto the gold surface from thin vapor deposited layers on SAMs have been described.20,21 The substitution reaction with the silver ions could not be quantified because the shift in binding energy between the different states of silver and carbon was indistinguishable. The sodium substituted SAM shows an ESCA composition and Na/O ratio similar to that of the silver substituted SAM (Table 1). Monolayers substituted with copper ions showed that copper was present at all takeoff angles (Table 1). However, the amount of copper was roughly a third of that of silver. Elemental composition of the substituted monolayer was in agreement with the results described above. Again, the constant amount of copper at all takeoff angles suggests a similar double-layer structure as described for the silver ions. Copper is known to form a dimeric bond with two carboxyl groups such as {COO-2Cu2+},19 whereas silver forms a single ionic bond {COO-Ag+}.22 This contributes to the lower copper ion concentration compared to silver. Current work is focusing on the oxidation state of the copper by high-resolution angle-dependent ESCA analysis (20) Herdt, G. C.; Jung, D. R.; Czanderna, A. W. Prog. Surf. Sci. 1995, 50, 103-129. (21) Jung, D. R.; Czanderna, A. W. J. Vac. Sci. Technol. A 1995, 13, 1337-44. (22) Reilley, C. N.; Everhart, D. S. ESCA Analysis of Functional Groups on Modified Polymer Surfaces; Wiley-Interscience: New York, 1982.

in order to determine the exact level of -COO- substituted copper ions. However, as will be shown later, the metal ions adsorbed to the gold do not contribute to the cationization and can be neglected. Angular dependent ESCA studies on a thallium substituted monolayer were carried out as well (Table 1). Thallium is known to form a strong bond with carboxylic groups23 and exhibits a much lower affinity for the gold surface than Na, Ag, and Cu, which are known to be natural contaminants of gold. The thallium signal increases as a function of the takeoff angle. Calculation of the Tl:O ratio at all takeoff angles is constant, assuming little thallium adsorption onto the gold surface. The ratio also reveals a near stoichiometric ratio of the thallium to carboxyl groups, assuming that most of the hydrogen is replaced by thallium during the exchange process. Time-of-Flight SIMS Analysis. Negative ToF-SIMS spectra of carboxylic acid terminated SAMs were recorded and revealed distinct peaks at m/z 287.2 (M-H)- and 681.2 (Au2(M-H)-) with (M) describing the mercaptohexadecanoic acid molecule [HS(CH2)15COOH]. Thiol fragments bound to gold such as AuSC2H2- and Au2SC2H2- were some of the major peaks in the spectra. Additionally, strong peaks were recorded at m/z 197 (Au-), 229 (AuS-), 394 (Au2-), 426 (Au2S-), and 458 (Au2S2-). Comparison of the spectra obtained from the COOHSAM and the substituted COO-Ag+-SAM shows similar peaks in the low-mass region, yet some distinct differences were found for peaks at higher masses (m/z >200). These differences include the silver ions detected at m/z 106.9 (Ag+) and its isotope at m/z 108.9 (Ag+) at a ratio of approximately 1:1 (52% and 48%, respectively). Figure 1, bottom row, shows that the metal (X) ion modified monolayer displayed peaks for both the unreacted (MH)- (c) and the ion substituted (MX-H)- (d) mercaptohexadecanoic acid molecules. Analysis of the unreacted monolayer revealed the molecule (M-H)- (a), but no peaks (23) Batich, C. D. Appl. Surf. Sci. 1988, 32, 57-73.

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Figure 2. Positive ion ToF-SIMS spectrum of PEO(400 kDa) deposited onto a carboxylic terminated self-assembled monolayer.

Figure 3. Positive ion ToF-SIMS spectrum of PEO(400 kDa) deposited onto a sodium ion substituted carboxylic terminated self-assembled monolayer.

assigned to a Ag+ substituted thiol molecule (b) were found. In addition, differences included cationized hydrocarbon species such as AgC2H4- and metal clusters such as AuAg-. Quantification of the ion substitution was not possible because the (M-H)- molecule might have occurred from both unreacted thiol molecules or metal ion substituted molecules that lost the silver ion during the emission process. Positive ToF-SIMS spectra were acquired to monitor the fragmentation of poly(ethylene oxide) (PEO). Positive ion spectra yield more information for the characterization of cationized polymers than negative spectra since the cationized fragments are positively charged from the metal ion. Both 1 and 400 kDa PEO deposited on an unsubstituted carboxylic terminated SAM were analyzed by ToF-SIMS. In the low-mass region, a variety of peaks (C+, CH+, CH+2, C2H+, C2H3+, C3H5+, and C3H7+) revealed little structural information. They occurred from either the monolayer, the deposited polymer, or hydrocarbon contamination. Fragments at m/z 43.02 (-C2H3O-)+, 45.03 (-C2H5O-)+,

and 73.03 (-C3H5O2-)+ were assigned as the characteristic fingerprint of the deposited PEO. Other peaks present in the spectrum were related to the SAM and gold substrate. Larger fragments (n > 3 monomer units) of the PEO were not detected (Figure 2). The spectra of 400 kDa PEO deposited onto metal ion substituted carboxylic terminated SAMs showed considerable differences (Figure 3). In the low-mass region (m/z < 200) of the spectra, peaks similar to the unsubstituted sample were seen, with additional peaks describing the metal ions. The high-mass region, however, exhibited a peak distribution that was found to be multiple monomer and submonomer fragments cationized with the respective metal ion. Figure 3 shows the high-mass region (m/z > 400) of PEO 400 kDa deposited onto a sodium ion substituted SAM, where the peaks were resolved to be multiple PEO monomer units cationized with a Na+ ion. The difference in mass between two major peaks was calculated to be m/z 44.03 (CH2CH2O), which is the equivalent mass value of one PEO monomer unit. The cationized n-monomer fragments are described as Rn )

Substrates for Polymer Cationization

Figure 4. Fragmentation pattern of sodium cationized PEO(400 kDa) positive ion ToF-SIMS spectrum.

Na+-(CH2CH2O)n-. Fragments of up to 30 repeat units cationized with the Na+ ion could be detected. Other peaks in this high-mass region were attributed to subfragments of the monomer units. Specific subfragmentation patterns were found for Rn to Rn-1. The different peaks were assigned, and the pattern illustrated in Figure 4 was obtained. Sequential extraction of two methylene groups and one oxygen is seen in Figure 4a. The resulting fragment corresponds exactly to Rn-1. Interestingly, the reversed order with oxygen extraction followed by two methylene groups is detected as well (Figure 4b). This indicates that the metal ion did not necessarily cationize at an end oxygen, but with any oxygen within the fragment. The extraction of two hydrogen (Figure 4c) results in a double bond that seems favorable for the stability of the compound since the peak intensity is comparable to or even higher than the n-monomer fragment. From the PEO fragment containing one double bond (Figure 4c), further extraction of a methylene group, a carbon, and ultimately an oxygen leads to Rn-1 as well (Figure 4d). The reversed extraction, removal of an oxygen, followed by a carbon, and a methylene group was also observed (Figure 4e). This double bond will most likely be at one end of the polymer chain in order to stabilize the free electron. The described pattern is found for all cationized PEO fragments with n > 6 and n < 30. Cationization of 400 kDa PEO showed large chain fragments that originate from chain scission of the entangled polymer chains (Figure 3). Desorption of a whole polymer molecule with the mass 400 kDa would not be anticipated and was not observed with ToF-SIMS. However, 1 kDa PEO polymer molecules consisting of about n ) 23 monomer units were detected. In the high-mass range, a different peak distribution was observed for spectra acquired of PEO(1 kDa) compared to PEO(400 kDa) deposited on metal ion substituted SAMs. The largest detected fragments showed masses of approximately 1.5 kDa. Figure 5 displays a spectrum of 1 kDa PEO deposited onto silver substituted carboxylic terminated SAMs. The overall spectrum at higher masses (m/z > 400) shows two peak distributions with maxima at 0.5 and 1 kDa. The peaks at approximately 1 kDa correspond to the whole PEO molecule (n ) 23) cationized

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with the metal ion. The Gaussian peak pattern from 0.5 to about 1.5 kDa represents the molecular weight distribution of the PEO(1 kDa), which is calculated to be Mw/Mn ) 1.16, typical for the type of polymer used. The second peak distribution starting at 0.4 kDa resulted from fragmented PEO molecules cationized with the metal ion ()R), as observed also on the 400 kDa PEO. This distribution exhibited a maximum around 0.5 kDa and decreased slowly up to 1.6 kDa. The difference between the fragments, R, and the molecules, M, is in the terminal groups. While whole PEO molecules contain both the hydroxyl and hydroxide end group, the fragments only consist of the terminated monomer units. Scission of the whole molecules occurs between the monomer units, leaving the fragments without terminal groups. The mass difference of the whole desorbed molecule Mn and the fragmented species Rn is exactly 18.01 m/z (H2O). As described above, this difference could not be observed on the 400 kDa PEO as only fragmented species were observed. A comparison of the metal ions studied here in terms of cationization yield is shown in Figure 6. PEO(1 kDa) was spin-cast on carboxylic terminated self-assembled monolayers that had previously been immersed in various metal ion containing solutions. The yield of cationized species for the different metal ions was determined by normalizing the value of the integrated peak area of X+(CH2CH2O)23 ion by the value of the integrated peak area of the CH2CH3O+ ion. With the exception of barium, all other metal ions that were investigated cationized the PEO. It seems that barium does not form a bond with the carboxylic groups or is so weakly bound that it is removed during rinsing of the substrate. This is consistent with observations that describe the absence of barium after competitive adsorption to carboxylic groups.24 The intensities of silver, copper, and thallium cationized PEO fragments in the high-mass area were comparable, while lithium and sodium displayed peaks that were considerably more intense. Copper ions are known to form a strong dimeric bond with the carboxyl groups when substituting the acidic hydrogen.19 Both thallium and silver ions are described as forming strong bonds with the carboxylic groups as well.23,25 The sodium and lithium ions are smaller and more hydrogen-like than copper, silver, and thallium ions. It has been shown that they form weak bonds with the carboxyl groups that can be removed with extensive washing.26,27 Therefore, we suggest that it is more desirable to work with metal ions weakly bound to the carboxyl groups. The weak bond to the carboxyl group enables the surface ion to break free upon SIMS primary ion impact and cationize the polymer and its fragments. Count intensities for Na+ and Li+ cationized polymer fragments were compared to results obtained from control experiments carried out on etched silver substrates. While the intensities of the cationized polymer fragments were comparable, the ratio of cationized fragments to the total metal ion intensity was much higher for the SAM substrates. Unfortunately, it was not possible to measure the metal-carboxy bond energies and relate the strengths to the cationized polymer peak signal intensities. The (24) Czuha, M.; Riggs, W. M. Anal. Chem. 1975, 47, 1836-1838. (25) Batich, C. D.; Wendt, R. C. Chemical Labels to Distinguish Surface Functional Groups using X-Ray Photoelectron Spectroscopy; Miami, 1981; Vol. 162, pp 221-235. (26) Everhart, D. S.; Reilley, C. N. Anal. Chem. 1981, 53, 665-676. (27) Hammond, J. S.; Holubka, J. W. On the Focus of Paint Adhesion Failure in Corrosion; Miami, 1978.

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Figure 5. Positive ion ToF-SIMS spectrum of PEO(1 kDa) deposited onto a silver ion substituted carboxylic terminated selfassembled monolayer.

Figure 6. Positive ion TOF-SIMS spectrum of a reference CH3 terminated SAM, dipped in 10 mM AgNO3-containing solution for 1 min, with subsequent PEO(1 kDa) deposition.

interaction between COO-, the metal cation, atmospheric water present, and the PEO overlayer is most likely complex and cannot be described on the basis of our results. To rule out cationization from metal ions that were adsorbed to the gold surface, methyl terminated SAMs were prepared in the same way as described for the carboxylic terminated SAM. When the monolayer is exposed to the metal ion solution, the methyl-functionalized headgroup is inert to reaction with the metal ions. However, as described above in the ESCA section, some of the metal ions will adsorb to the gold surface. Positive ToF-SIMS spectra of a methyl terminated self-assembled monolayer were acquired after dipping the sample in a silver ion containing solution and washing (Figure 7). Since silver is known to have a high affinity for gold, it can be expected that the high concentration of silver ions in the solution would lead to silver adsorption at the gold interface. In the ToF-SIMS spectra, strong silver signals at 106.91 m/z and 108.91 m/z were detected. The intensities

Figure 7. Plot of the relative yield of cationized species from the various metal ions. The Y-axis is defined by the counts of the metal ion cationized whole PEO molecule (X+ H(CH2CH2O)23OH), normalized by the value of the integrated peak area of the CH2CH3O+ ion.

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were similar to that obtained on the carboxyl terminated SAM. In the high-mass region (400-2000 m/z), however, there was no indication of cationized fragments. Silver ions desorbed from the gold interface did not cationize polymer fragments or thiols from the SAM. Conclusions We demonstrated that metal ions bound to carboxylic terminated self-assembled monolayers cationize large polymer fragments in ToF-SIMS. PEO(400 kDa) deposited on a metal ion substituted SAM displayed high counts of cationized polymer fragments. PEO(1 kDa) exhibited cationized fragments and additional whole polymer fragments that represent the molecular weight distribution of the polymer. Lithium and sodium displayed the highest count rates of cationized 1 kDa PEO molecules. The count rate appears to be a function of the binding strength of the metal ion with the carboxylic group. This suggests that binding of the metal ions to the surface should be

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sufficiently weak, so that ions can break free and cationize the polymer fragments. Metal ions that are not in close proximity to the polymer, such as the free metal that is bound to the gold in a nonspecific reaction, do not cationize polymer fragments when desorbed from the surface. The substrates used in this work are easy to prepare and reproducible. This model system may be used for further studies to understand the mechanism of cationization in ToF-SIMS experiments. Acknowledgment. This work was supported by UWEB (University of Washington Engineered Biomaterials, UWEB, NSF EEC-9529161) and the National ESCA and Surface Analysis Center for Biomedical Problems (NESAC/BIO NIH NCRR grant RR01296). Special thanks to Marcus Textor and Dave Castner for stimulating discussions. LA991436Z