ARTICLE pubs.acs.org/JPCC
Adsorption of Mixed Peptide/Thiol Adlayers on InAs: Assessment of Different Functionalization Strategies Using X-ray Photoelectron Spectroscopy Scott Jewett,† Dmitry Zemlyanov,‡,§ and Albena Ivanisevic*,†,|| Weldon School of Biomedical Engineering, ‡Birck Nanotechnology Center, and §Department of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907, United States Materials Science and Engineering, North Carolina State University, Joint Department of Biomedical Engineering NCSU/UNC-CH, Raleigh, North Carolina 27695, United States
)
†
bS Supporting Information ABSTRACT: Mixed adlayers are commonly used to create surfaces with multiple functional groups or molecular gradients. The resulting surfaces are designed to have the combined effects of each adsorbed functional group. In this work, we combine the passivation ability of a thiol with the functionality of a peptide by assembling mixed peptide/ thiol adlayers on InAs. Both sequential (insertion) and simultaneous (co-assembly) methods are used to functionalize InAs and are analyzed by contact angle measurements and X-ray photoelectron spectroscopy (XPS). Peptide coverage, thiol coverage, and oxide layer thicknesses are estimated from XPS data. For each method, oxide layer thicknesses were significantly lower than the oxide thicknesses for as-received samples. Surfaces functionalized using the insertion methods had lower oxide layer thicknesses than peptide only samples, indicating that the addition of the thiol helped block oxide regrowth. Functionalization with the thiol followed by the peptide yielded surfaces with the highest peptide coverage and most likely formed a layered peptide/thiol structure. Functionalization with the peptide followed by the thiol produced surfaces with a lower peptide coverage and most likely consisted of peptide clusters surrounded by adsorbed thiol. Surfaces functionalized by the co-assembly method were predicted to form a disordered peptide/thiol adlayer.
’ INTRODUCTION For IIIV semiconductors such as InAs, chemical modification of the surface can accomplish two critical functions: passivation of the surface and functionalization with target molecules of interest. A key aspect of passivation is the prevention of oxide growth, which stabilizes the surface and can prevent the dissolution of potentially toxic molecules in aqueous conditions, which is critical for biological applications.1 In certain instances, a single adsorbate cannot accomplish both passivation and functionalization; therefore, it can be advantageous to prepare surfaces composed of more than one different molecule. Mixed self-assembled monolayers (SAMs) have emerged as a simple and robust method of creating well-defined molecular gradients on a surface. Mixed SAMs have been previously assembled on semiconductors such as silicon,2,3 GaAs,4,5 and InP6 and on noble metals such as gold710 and silver.11 InAs is a IIIV semiconductor with a small band gap (0.354 eV) and extremely high electron mobility in the near-surface region. Because of its unique electronic properties, InAs is an attractive material for a variety of applications, including biological applications such as direct cell biosensing12 and biomolecule detection using Hall sensors.1315 However, in biological applications the use of InAs is limited due to its inherent toxicity. The adsorption of passivating layers has been shown to decrease r 2011 American Chemical Society
the toxicity of a similar IIIV semiconductor, GaAs, by preventing the corrosion of toxic components from the oxide layer.1,16 Additionally, these studies suggest that as more layers are adsorbed onto the surface fewer toxic effects are observed. Adsorbing biological molecules such as amino acids and peptides onto semiconductors has been of interest for the past decade.1719 Such biological molecules could be used as receptors for biosensors or serve as templates for the low temperature of growth nanostructures.20,21 In terms of passivation, such biological molecules have been shown to have limited effectiveness.2224 Our lab has previously shown that a collagen binding peptide (GCGGELYKSILY) can bind tightly to InAs, but the peptide has limited ability to prevent the regrowth of the oxide layer.24 As with most IIIV semiconductors, passivation of InAs using thiol-based molecules has been shown to be successful in a number of studies.2529 A mixed adlayer consisting of passivating thiol molecules along with the functional biomolecules could potentially passivate the surface while maintaining functionality. Received: April 6, 2011 Revised: June 14, 2011 Published: June 20, 2011 14244
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Figure 1. Molecular structure of (A) thioglycolic acid (TGA) and (B) mercaptopropanoic acid (MPA).
Scheme 1. Summary of the Three Different Functionalization Methods Used for the Formation of Mixed Peptide/Thiol Adlayers on InAs
In this report, we assess the formation of mixed peptide/thiol adlayers on InAs with the goal of eventually decreasing the toxicity of InAs for biological applications. The mixed adlayers consist of two different thiols, mercaptopropanoic acid (MPA) and thioglycolic acid (TGA) (Figure 1), along with a collagen binding peptide (GCGGELYKSILY). The peptide was chosen because it has been shown previously to adsorb onto InAs,24 and it could have future applications for increasing biocompatibility of InAs surfaces by promoting the adsorption of collagen.30 The order and method in which the functionalization is performed play a critical role in the resulting adlayer properties.8 Therefore, we explore three different functionalization methods to create the mixed adlayers (Scheme 1). Contact angle analysis and X-ray photoelectron spectroscopy (XPS) are used to characterize the formation of each adlayer. Quantitative analysis of the XPS data is used to estimate the peptide coverage, thiol coverage, and oxide layer thicknesses of each different functionalization method.
’ EXPERIMENTAL METHODS Surface Functionalization. Substrates and Chemicals. The surfaces used in this report were undoped InAs(100) substrates with surface areas ∼0.5 cm2. Thioglycolic acid (TGA), mercaptopropanoic acid (MPA) (Figure 1), and 99% bromine were purchased from Sigma-Aldrich and diluted from the stock solutions. The peptide was a collagen binding peptide (sequence: GCGGELYKSILY) and was synthesized using solid-phase synthesis as described previously.31 All water was of Milli-Q quality. Surface Cleaning and Etching. InAs samples were first cleaned by sonicating for 15 min in water, methanol, and ethanol and then dried with nitrogen. The native oxide layer was then removed by incubating cleaned samples for 3 min in a 0.1% bromine solution diluted in methanol. To minimize reoxidation,
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the freshly etched samples were immediately transferred to the XPS analyzer after etching. Insertion Method 1. Freshly etched samples were immediately transferred to a 100 μM TGA or MPA solution diluted in water and incubated for six hours. Samples were then rinsed with water and dried with nitrogen. Samples were then transferred to 100 μM peptide solution and incubated for two hours. Samples were then rinsed with water, dried with nitrogen, and stored at room temperature for three days until analysis. Insertion Method 2. Freshly etched samples were immediately transferred to 100 μM peptide solution and incubated for two hours. Samples were then rinsed with water and dried with nitrogen. Samples were then transferred to a 100 μM TGA or MPA solution diluted in water and incubated for six hours. Samples were then rinsed with water, dried with nitrogen, and stored at room temperature for three days until analysis. Co-Assembly Method. Freshly etched samples were immediately transferred to a solution containing 100 μM TGA or MPA and 100 μM peptide diluted with water and incubated for six hours. Samples were then rinsed with water, dried with nitrogen, and stored at room temperature for three days until analysis. Scheme 1 outlines the order of functionalization for each assembly method. Contact Angle. Contact angles for each sample were determined using a Tantec CAM-PLUS contact angle meter. Four measurements were performed on each sample, and angles were determined using the half-angle method. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) was the primary technique used to characterize the surfaces in this report. Other common techniques such as ellipsometry and Fourier transform infrared spectroscopy (FTIR) can also be used for surface characterization. Our group has previously attempted both ellipsometry and FTIR experiments on bromine-etched InAs; however, there were difficulties in obtaining reproducible results due to the formation of rough surfaces after the bromine etch. XPS data were obtained with a Kratos Ultra Axis DLD spectrometer using Al KR monochromatic X-ray radiation (1486.6 eV). Survey and high-resolution spectra were collected at pass energies of 160 and 20 eV, respectively. Between two and nine scans were performed for each sample to obtain a reasonable signal-to-noise ratio. The spectra were taken at angles of 0°, 30°, 45°, and 60° with respect to the surface normal. The high-resolution spectra were collected for the As 2p, O 1s, In 3d, N 1s, C 1s, S 2p, As 3d, and In 4d core levels. Charge correction was performed using the AsIn peak at 40.78 eV in the As 3d spectra. All data were analyzed using the CasaXPS software, version 2.3.15. GaussianLorentzian line shapes were used to fit the data, and the background was modeled using either Shirley or Linear functions. Quantification of XPS Data: Coverage and Oxide Thickness Calculation. Adlayer Coverage. The N 1s and S 2p spectra were used to calculate peptide and thiol coverage for each sample. The adlayers are assumed to be a nonattenuating overlayer as outlined by Fadley,32 and the coverage can be calculated using the following equation (for complete derivation, see Supporting Information). Θ¼
dσs cosðθÞ Λsubst ðEs Þ Nl ðθÞ e dσ l dΩ d Ns ðθÞ dΩ
ð1Þ
In this equation, the coverage, Θ, is defined as the number of peptide or thiol molecules per either indium or arsenic atoms on the surface; 14245
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Figure 2. As 2p3/2, In 3d5/2, and As 3d spectra of InAs samples functionalized using insertion method 1.
θ is the photoemission angle between the surface normal and the direction of electron emission; (dσs)/(dΩ) is a differential crosssection for In 3d or As 3d photoemission lines depending on the tabulated Scofield relative sensitivity factor corrected for the Reilman asymmetric parameter; (dσ1)/(dΩ) is a differential cross-section of the N 1s or S 2p photoemission lines; Λesubst is the inelastic mean free pass (IMFP) of the In 3d or As 3d photoelectrons through InAs; Nl(θ) is the intensity of the N 1s or S 2p spectra; Ns(θ) is the intensity of the InAs component in the In 3d peak or the AsIn component in the As 3d spectra; and d is the distance between the closest plane of In/As(100), which is half of the lattice constant of InAs, 3.03 Å. As discussed by Jablonski and Powell,33 the IMFPs can be replaced with electron attenuation lengths for quantitative analysis (QEALs) as calculated by the NIST SRD 82 software.34 Oxide Layer Thickness. The oxide thickness on the InAs samples was approximated by modeling the surfaces as a semiinfinite substrate with a uniform overlayer as outlined by Fadley32 using the following equation (see Supporting Information for complete derivation) t ¼
Λoxide ðEs ÞcosðθÞ e
N oxide ðθÞ Fsubst Λsubst ðEs Þ e ln +1 oxide N subst ðθÞ Foxide Λe ðEs Þ
!
ð2Þ In this equation, Noxide is the intensity of the In2O3, As2O3, or As2O5 components in the In 3d and As 3d spectra; Nsubst is the
intensity of the of InAs component in the In 3d spectra or the intensity of the AsIn component in the As 3d spectra; Foxide is the atomic density of indium or arsenic atoms present in In2O3, As2O3, or As2O5; Fsubst is the atomic density of indium or arsenic present in the substrate; t is the thickness of the uniform oxide overlayer; and Λeoxide(Es) and Λesubst(Es) are the IMFPs of the In 3d or As 3d photoelectrons through the oxide layer and InAs, respectively, which are replaced by the QEALs as calculated by the NIST SRD 82 software.34 The oxide layer is a composite consisting of three distinct components, As2O3, As2O5, and In2O3. Therefore, the QEALs are adjusted to account for the different concentration of oxide components using the following equation 2 O3 2 O5 2 O3 ðEs Þ ¼ RΛAs ðEs Þ + βΛAs ðEs Þ + γΛIn ðEs Þ ð3Þ Λoxide e e e e
In this equation, R, β, and γ are correction factors that represent percent concentrations of each oxide component as calculated from each sample’s As 3d and In 3d spectra. Also, oxides were assumed to be homogeneously blended, so Foxide was also adjusted using the correction factors.
’ RESULTS AND DISCUSSION Analysis of Mixed Adlayer Formation and Composition. Clean and Freshly Etched Samples. In ambient conditions, InAs
forms a native oxide layer consisting of indium and arsenic oxide 14246
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Table 1. Average Contact Angles avg. contact angle (deg.)a
Figure 3. Representative S 2p spectra. The S 2p spectra of all samples containing thiols were similar. The peaks represent SIn binding. a
components. In this report, clean, as-received InAs was assumed to be fully oxidized, and the resulting XPS spectra were used to determine the approximate peak positions of the oxide components for all subsequent samples. On the basis of the As 2p3/2, In 3d5/2, and As 3d5/2 spectra of the clean sample (Figure 2), the native oxide layer consists of three separate components, In2O3, As2O3, and As2O5. In the In 3d5/2 spectrum, In2O3 is present at ∼445.2 eV, along with the InAs component at 444.3 eV. The As2O3 and As2O5 components are observed in the As 3d spectrum at 44.1 and 45.5 eV, respectively, along with the AsIn component at 40.8 eV. The As 2p3/2 spectrum shows only a single AsxOx oxide component present at 1326.3 eV, along with the AsIn component at 1322.3 eV. The As 2p3/2 spectra were collected due to high surface sensitivity. All of the peaks mentioned above are at comparable positions to previous studies on InAs.22,25,27 The removal of the native oxide layer is essential to take advantage of the electronic properties of InAs. In this report, a bromine etch was used to remove the native oxide layer on InAs. On the basis of the As 2p3/2, In 3d, and As 3d spectra of the freshly etched sample, there was a significant decrease in In2O3, As2O3, and As2O5 oxide components after etching (Figure 2), when compared with a clean InAs sample, indicating that the bromine etch successfully removed the native InAs oxide layer. Some oxide components are detected in the freshly etched sample; however, it was assigned to reoxidation during sample transfer through air for XPS analysis. Importantly, there was no lag time between etching and functionalization, so a minimal amount of oxide was present during thiol or peptide functionalization. Insertion Method 1. We used three different functionalization methods to assemble a mixed peptide/thiol adlayer on InAs. Insertion method 1 consisted of an initial thiol adsorption (either MPA or TGA), followed by peptide adsorption. The two thiols MPA and TGA were chosen because their solubility in water allows them to be easily used in an aqueous environment, which is a necessity for the peptide. Previous reports suggest alkanethiols bind to InAs primarily thorough InS bonds.2527,35 Examination of the S 2p spectra (Figure 3) shows a peak at ∼162.1 eV, which is similar to previous reports of thiol molecules bound to InAs,27,29 suggesting that the thiols are present and bound to the InAs surface. The In 3d spectra are often used to confirm InS binding, but since InS and InO binding energies are less than 0.5 eV apart it can be difficult to definitively show InS binding solely through the In 3d spectra. However, there is no evidence of AsS binding, based on the lack of a component at 43 eV in the As 3d spectra. There was also a
clean
59.0 ( 3.2
freshly etched
39.8 ( 3.4
peptide only (insertion 2)
65.3 ( 1.9
TGA only (insertion 1)
40.8 ( 2.2
MPA only (insertion 1)
51.5 ( 3.4
TGA and peptide (co-assembly)
51.8 ( 2.1
MPA and peptide (co-assembly)
54.3 ( 3.3
TGA/peptide (insertion 1) MPA/peptide (insertion 1)
54.8 ( 3.2 62.5 ( 2.6
peptide/TGA (insertion 2)
59.8 ( 2.1
peptide/MPA (insertion 2)
59.0 ( 2.2
Results are in the form mean ( st. dev.
Figure 4. N 1s spectra of InAs samples functionalized using insertion method 1.
depletion of arsenic in the near surface region based on indium and arsenic elemental concentrations (data not shown), making it much more likely that sulfur bound to indium as opposed to arsenic. The average contact angle of the TGA-only sample is similar to the average contact angle of the freshly etched sample and lower than the average contact angle of the clean sample (Table 1), suggesting adsorption with the carboxylic acid groups facing outward. The average contact angle for the MPA-only sample is higher than the freshly etched sample but still lower than the clean sample, indicating that the organization of the thiol molecules on the MPA-only sample may be more disordered in structure than the TGA-only sample. Also, there is a significant increase in the carboxylic acid component at ∼288.5 eV in the C 1s spectra of the TGA- and MPA-only samples when compared with clean and freshly etched samples (data not shown). The second step in insertion method 1 consists of incubation in a 100 μM peptide solution. Figure 4 shows the N 1s spectra for the samples prepared using insertion method 1. The N 1s spectra for the peptide-containing samples have a dominant peak at ∼400 eV, which is indicative of amide groups present in the peptide. There is also a distinct shoulder present at ∼398 eV, which is indicative of NH2 groups present on the peptide. A more detailed analysis of the structure of the adsorbed adlayer will be presented in subsequent sections. It is worth noting that an alternative explanation for the peak at ∼398 eV could be nitrogen bound to an inorganic substance, as 14247
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Figure 5. As 2p3/2, In 3d5/2, and As 3d spectra of InAs samples functionalized using insertion method 2.
seen with the low binding energies in the NGa component in GaN.36 There have been previous suggestions of individual amino acids binding to InAs through nitrogen groups;23 however, the binding of nitrogen to indium or arsenic is thermodynamically unfavorable and in most cases would require additional heat or solvents to overcome the energy barriers required for binding. To test if the component at ∼398 eV was indicative of nitrogen bound to indium or arsenic, we calculated the ratios between the NH2 and amide intensities for the different photoemission angles because the higher photoemission angles are more representative of the outermost surface molecules. For the 0° photoemission angle, the ratios are between 9 and 35%, which are relatively similar to the expected value based on peptide molecular structure of 15%. As the photoemission angle increases, the ratios increase, indicating that the component at ∼398 eV is closer to the surface and is not indicative of what is happening at the InAs/peptide interface. Therefore, the component at ∼398 eV represents NH2 groups and not nitrogen bound to indium or arsenic. Insertion Method 2. Insertion method 2 consists of an initial adsorption of a peptide adlayer, followed by incubation in either TGA or MPA solution. The As 2p3/2, In 3d5/2, and As 3d5/2 spectra are similar to insertion method 1 and can be seen in Figure 5. As seen in Figure 6, on the peptide-only surface, there is a dominant peak at ∼400 eV, along with a shoulder present at ∼398 eV, indicative of amide bonds and NH2 groups,
Figure 6. N 1s spectra of InAs samples functionalized using insertion method 2.
respectively. On the basis of our lab’s previous work, the peptide GCGGELYKSILY adsorbs in disordered clusters on InAs surfaces.24 Peptide cluster formation has been observed on other semiconductor surfaces with a variety of different peptides.18,19 The formation of clusters on the surface is due to the higher affinity of peptide/peptide interactions when compared to peptide/semiconductor interactions.19 In this study, increases in contact angle from ∼59° for a clean surface to ∼65° provide 14248
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Figure 7. As 2p3/2, In 3d5/2, and As 3d spectra of InAs samples functionalized using the co-assembly method.
additional evidence of peptide adsorption in disordered clusters onto the InAs. It is worthwhile to point out that the NH2 component did not appear in this previous study; however, the discrepancy is due to the differences in the amount of peptide adsorbed on the surface. In the current study, samples were incubated in a 100 μM peptide solution as opposed to a 10 μM solution used previously. The amount of peptide adsorbed on semiconductors has previously been directly related to the concentration of the peptide solution, with saturation occurring at peptide concentrations greater than ∼20 μg/mL.18 Therefore, a greater amount of peptide adsorbed on the InAs surface in this study, allowing direct observation of the NH2 component. After subsequent thiol adsorption, the S 2p spectrum shows a peak at ∼162.1 eV, indicating that the thiol is present on the surface and bound to indium through InS bonds as discussed above. Since the peptide adsorbs in clusters on the surface, the thiol most likely adsorbed in spots where the peptide was not adsorbed. The presence of the thiol is further confirmed by the decrease in contact angle when comparing peptide-only to peptide/thiol-functionalized surfaces (Table 1). Interestingly, after the adsorption of the thiol, the NH2 component is no longer visible. Possibly, the carboxylic acid groups in the thiol were able bind to the NH2 groups, effectively creating amide bonds with a binding energy of ∼400 eV.
Figure 8. N 1s spectra of InAs samples functionalized using the coassembly method.
Co-Assembly. The co-assembly method consists of a simultaneous adsorption of a thiol and peptide in an aqueous solution. In general, the As 2p3/2, In 3d5/2, and As 3d5/2 spectra are similar to insertion methods 1 and 2 (Figure 7). On the basis of the S 2p and N 1s spectra, (Figures 3 and 8), both the thiol and peptide are present on the surface. However, it is difficult to determine the 14249
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Table 2. Estimated Peptide and Thiol Coverages peptide coveragea molecules per In atom
molecules per As atom
molecules per In atom
molecules per As atom
(N 1s/In 3d)
(N 1s/As 3d)
(S 2p/In 3d)
(S 2p/As 3d)
0.38 ( 0.10
0.29 ( 0.03
TGA only (insertion 1)
1.04 ( 0.21
1.27 ( 0.10
MPA only (insertion 1)
0.96 ( 0.21
1.21 ( 0.20
peptide only (insertion 2)
a
thiol coveragea
TGA and peptide (co-assembly)
0.29 ( 0.05
0.33 ( 0.03
1.26 ( 0.28
1.40 ( 0.18
MPA and peptide (co-assembly)
0.18 ( 0.03
0.22 ( 0.02
0.89 ( 0.16
1.09 ( 0.08
TGA/peptide (insertion 1)
0.34 ( 0.04
0.38 ( 0.01
1.18 ( 0.24
1.31 ( 0.08
MPA/peptide (insertion 1) peptide/TGA (insertion 2)
0.37 ( 0.07 0.18 ( 0.04
0.40 ( 0.03 0.22 ( 0.03
1.12 ( 0.21 1.01 ( 0.26
1.20 ( 0.13 1.22 ( 0.22
peptide/MPA (insertion 2)
0.13 ( 0.03
0.17 ( 0.02
0.99 ( 0.19
1.12 ( 0.15
Coverage was averaged using the 0°, 30°, 45°, and 60° photoemission angles and is in the form mean ( st. dev.
exact layout of the surface because of the simultaneous nature of the assembly. Previously, co-assembled mixed adlayers have been shown to be relatively disordered in structure.8 Here, the average contact angles are lower than the contact angles seen for insertion methods 1 and 2 and similar to thiol-only samples, indicating that a greater amount of the thiol is adsorbed on the surface than peptide. However, it should be noted that the difference between contact angles is relatively small. Due to slight differences in surface structure and orientation which can occur during the bromine etch, there is inherent variability with contact angle measurements, making it difficult to make chemical conclusions based solely on contact angle analysis. Estimated Peptide and Thiol Coverage. The N 1s and S 2p spectra can be used to calculate the peptide and thiol coverage, respectively, using eq 1. Table 2 summarizes the approximate peptide and thiol coverage as calculated by both the In 3d and As 3d photoemission lines. For both the peptide and thiol coverage, the coverage calculated by the As 3d spectra is slightly higher than the In 3d coverage. This discrepancy is caused by a consistent arsenic depletion seen in the near surface region for all samples. Such arsenic depletion leaves an indium-rich surface and is usually caused by the surface etch.23,35,37 The depletion of arsenic will therefore slightly inflate the estimated peptide and thiol coverage when calculated using the As 3d spectra. Overall, peptide coverage ranges from 13 to 40% of a monolayer, and thiol coverage ranges from about 0.9 to 1.4 monolayers. This thiol coverage fits well with the prediction of InS bonding and indicates that the thiols are able to form a relatively good monolayer on the InAs surface. Overall, the thiol coverage is relatively similar for all samples, and it is difficult to identify definitive trends based on thiol coverage results. However, some general trends can be observed. As seen in Table 2, samples containing TGA have consistently higher thiol coverage than samples containing MPA. This combined with the higher average contact angle seen for the MPA-functionalized sample suggests that TGA has a higher affinity for InAs, although the reason behind this is unclear. Also, the thiol coverage is higher for insertion method 1 when compared to insertion method 2, which further suggests that the thiol primarily adsorbs onto the InAs surface where the peptide is not clustered during insertion method 2. The co-assembly methods have thiol coverage from ∼0.9 to 1.1 monolayers for the MPA and peptide surface and from ∼1.3 to 1.4 for the TGA and peptide surface. As discussed above, it is difficult to tell exactly the nature of the adlayer because
of the simultaneous nature of the adsorption, but it is apparent that the thiol molecules are still able to adsorb onto the InAs surface with the co-assembly method. In terms of peptide adsorption, coverage for insertion method 1 was the highest of all samples, at 3440% of a monolayer. Since we determined above that the thiol forms a relatively good monolayer of InS bonds on the surface, the high peptide coverage suggests that the majority of the peptide adsorbed on top of the thiol adlayer. Previous groups used the insertion method to actually insert molecules into a formed monolayer;8 however, the size of the peptide most likely inhibited this direct insertion. Furthermore, it is highly unlikely that such a high amount of peptide could be inserted into the thiol adlayer. This high peptide coverage further suggests that the thiol is primarily adsorbed with the carboxylic acid groups facing outward, as the peptide would have a higher affinity toward the carboxylic acid groups when compared with InAs. There is no significant difference between the coverage calculated for MPA and TGA, indicating that the type of thiol does not affect the subsequent peptide coverage. Insertion method 2 consisted of an initial peptide adlayer, followed by the adsorption of a thiol. Calculated coverage for insertion method 2 ranges from 29 to 38% for the initial peptide adsorption, followed by a significant decrease in coverage to 1322% after the subsequent thiol adsorption. As discussed above, the peptide most likely adsorbed in clusters on the surface, and the thiol adsorbed wherever the peptide was not present. The adsorbed molecules on the surface are not static and are continuously being freely exchanged in the solution during functionalization. The decrease in peptide coverage could indicate that since the thiol has a high affinity for InAs surface it is able to adsorb onto InAs and take the place of peptide molecules during the second step of insertion method 2. Finally, the co-assembly method showed coverage of 2933% for the TGA/peptide surfaces and 1822% for the MPA/peptide surfaces. Overall, the coverage is in the range between insertion method 1 and 2. Passivation of InAs Using Mixed Adlayers. Surface passivation is critical for semiconductor device function, and thiol-based molecules have been used previously to passivate InAs.2528 The passivation ability of the mixed adlayers can be seen qualitatively by observing the presence of the In2O3, As2O3, and As2O5 components in the As 2p3/2, As 3d, and In 3d spectra (Figures 2, 5, and 7). As discussed above, for the clean sample obvious oxide components can be seen in each spectrum, and all 14250
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Table 3. Estimated Oxide Layer Thicknesses oxide layer thickness oxide layer thickness In 3d (Å)a clean freshly etched peptide only (insertion 2)
19
As 3d (Å)a 27
6
10
9(2
11 ( 2
TGA only (insertion 1)
5(2
10 ( 3
MPA only (insertion 1)
5(2
11 ( 3
TGA and peptide (co-assembly)
5(2
10 ( 3
MPA and peptide (co-assembly) TGA/peptide (insertion 1)
6(2 4(2
11 ( 2 7(2
MPA/peptide (insertion 1)
5(2
8(2
peptide/TGA (insertion 2)
4(1
7(2
peptide/MPA (insertion 2)
5(2
10 ( 3
a
The thicknesses for the clean and freshly etched samples were calculated using the 0° photoemission angle. The thicknesses for the remaining samples were averaged using the 0°, 30°, 45°, and 60° photoemission angles. Results are in the form mean ( st. dev.
functionalized samples show distinct decreases in oxide component intensity. The passivation ability can be quantified by calculating the oxide layer thicknesses using eq 2. Table 3 summarizes the oxide layer thicknesses for the InAs samples as calculated for both the In 3d and As 3d spectra. The values are averaged for the different photoemission angles, with the exception of the clean and freshly etched surfaces. The oxide thicknesses range from 4 to 27 Å and are generally higher for the As 3d spectra. The discrepancy between As 3d and In 3d results is due to arsenic depletion in the near-surface region. As expected, the highest oxide thickness is for the clean sample at 1927 Å, more than double the oxide thicknesses of the other samples. The freshly etched sample has an oxide thickness of 610 Å, indicating the bromine etch removed the oxide layer. There was contact with ambient air during transport to the XPS analyzer, so some oxidation of the sample was expected. Several conclusions can be made based on the oxide thicknesses seen in Table 3. For insertion method 1, the first step was functionalization with TGA or MPA. The average oxide layer thicknesses for the TGA- and MPA-functionalized samples are lower than the clean samples, indicating that the oxide regrowth was blocked. The second step of insertion method 1 consisted of adsorption of the peptide onto the thiol-functionalized InAs. After peptide adsorption, the oxide layer thicknesses decrease even further, indicating that the peptide adsorption aids in the blocking of the oxide regrowth. The first step in insertion method 2 consisted of the adsorption of the peptide onto InAs. Previous results show that peptides have some passivation ability on InAs but do not completely block oxide regrowth.24 Here, for the peptide-only sample, the oxide thicknesses are lower than the clean substrate but generally higher than the remaining samples, suggesting that the peptide can inhibit oxide regrowth, but not to the extent of a thiol. The second step of insertion method 2 is the adsorption of TGA and MPA onto peptide-functionalized InAs. The oxide thicknesses decreased after thiol functionalization when compared with peptide-only samples, indicating that the addition of the thiol helps prevent the regrowth of the oxide layer. For the co-assembly method, the oxide layer thicknesses were in general slightly higher when compared with surfaces
functionalized using insertion methods 1 and 2, indicating that the co-assembly method is not as effective at preventing oxide regrowth on InAs as the insertion methods. Previous results show that surfaces assembled using co-assembly are more disordered,8 and the disordered nature could have allowed oxide regrowth to occur at a higher rate than seen with the insertion methods. In this study, we did not directly assess the toxicity or biocompatibility of the mixed adlayers assembled on InAs. Future studies using cell culture assays and inductively coupled plasma mass spectrometry (ICP-MS) will be performed to more directly assess the toxicity and biocompatibility of the assembled surfaces.
’ CONCLUSIONS In this report, we characterized three different functionalization methods for the formation of mixed peptide/thiol adlayers on InAs. The functionalization methods included two distinct insertion methods, which differed by the order of adsorption steps, and a co-assembly method. For insertion method 1, the thiol bound to the surface via InS bonds, and the peptide then adsorbed on top of the thiol adlayer, creating a layered peptide/ thiol adlayer. For insertion method 2, the peptide adsorbed in clusters on the surface, and the thiol most likely bound to sites where the peptide did not adsorb. The co-assembly method most likely formed a disordered peptide/thiol adlayer on the surface. Quantitative analysis of the XPS data was used to calculate the peptide and thiol coverage using the N 1s and S 2p spectra. Peptide coverage was highest for the insertion method 1 and lowest for the insertion method 2 and ranged from 13 to 40% of a monolayer. Thiol coverage was similar for all samples, ranging from 0.9 to 1.4 monolayers. The passivation ability of the different adlayers was assessed by calculating the oxide layer thicknesses for the InAs samples. Insertion methods 1 and 2 showed oxide layer thicknesses that were lower than both asreceived and peptide-only samples, indicating that the insertion methods can help prevent the regrowth of the oxide layer on InAs while incorporating a biomolecular functionality on the surface. ’ ASSOCIATED CONTENT
bS
Supporting Information. Additional information on the quantitative analysis of the XPS spectra. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Tel.: 919-515-2377. Fax: 919-515-7724. E-mail: ivanisevic@ ncsu.edu.
’ ACKNOWLEDGMENT The authors thank Dr. Alyssa Panitch and Dr. John Paderi for donating the peptide used in this study. ’ REFERENCES (1) Kirchner, C.; George, M.; Stein, B.; Parak, W. J.; Gaub, H. E.; Seitz, M. Adv. Funct. Mater. 2002, 12, 266–276. (2) Fabre, B.; Hauquier, F. J. Phys. Chem. B 2006, 110, 6848–6855. (3) Liu, Y. J.; Navasero, N. M.; Yu, H. Z. Langmuir 2004, 20, 4039 4050. (4) Lee, K.; Nair, P. R.; Alam, M. A.; Janes, D. B.; Wampler, H. P.; Zemlyanov, D. Y.; Ivanisevic, A. J. Appl. Phys. 2008, 103, 114510. 14251
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