Purdue University Purdue e-Pubs Birck and NCN Publications Birck Nanotechnology Center 12-20-2011 Quantitative Analysis of the Functionalization of Gallium Phosphide With Organic Azides David Richards Purdue University Philip Luce Purdue University Dmitry Zemlyanov Birck Nanotechnology Center, Purdue University, [email protected] Albena Ivanisevic North Carolina State University Follow this and additional works at: http://docs.lib.purdue.edu/nanopub Part of the Nanoscience and Nanotechnology Commons Richards, David; Luce, Philip; Zemlyanov, Dmitry; and Ivanisevic, Albena, "Quantitative Analysis of the Functionalization of Gallium Phosphide With Organic Azides" (2011). Birck and NCN Publications. Paper 1131. http://dx.doi.org/10.1002/sca.21012 This document has been made available through Purdue e-Pubs, a service of the Purdue University Libraries. Please contact [email protected] for additional information. SCANNING VOL. 34, 332–340 (2012) C Wiley Periodicals, Inc. Quantitative Analysis of the Functionalization of Gallium Phosphide With Organic Azides DAV I D RICHARDS1,PHILIP LUCE1,DMITRY ZEMLYANOV2, AND ALBENA IVANISEVIC3 1 Weldon School of Biomedical Engineering, Purdue University, West Lafayette, Indiana 2 Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 3 Department of Materials Science and Engineering, North Carolina State University, Joint Department of Biomedical Engineering NCSU/UNC-CH, Raleigh, North Carolina Summary: Gallium phosphide (GaP) surfaces were coupled plasma mass spectrometry was used to eval- functionalized with two different molecules that uate the gallium concentration in the stability solu- contain an azide moiety at their terminus. Com- tions. While the functionalization with the organic pound 4-azidophenacyl bromide (4AB) is an aryl azides did not provide complete suppression of gal- azide with a bromine group at its opposite termi- lium leaching, both of the azides decreased the leach- nus that provides easy identification of the molecule’s ing by 10–50%. SCANNING 34: 332–340, 2012. C presence on the surface with x-ray photoelec- 2012 Wiley Periodicals, Inc. tron spectroscopy (XPS). O-(2-aminoethyl)-O-(2- azidoethyl)pentaethylene glycol (AAP) is a small poly(ethylene glycol) molecule with an amine group Key words: gallium phosphide, azide, at its opposite terminus. Atomic force microscopy functionalization, XPS, toxicity was used to identify the uniformity of the clean and functionalized GaP surfaces. Water contact angle re- vealed a more hydrophobic surface with AAP func- tionalization (33◦) and even more hydrophobic (53◦) Introduction with the 4AB functionalized surface compared to a clean surface (16◦). XPS confirmed the presence A considerable amount of effort in the past decades of each of the organic azides on the surface. XPS has been devoted to developing implantable devices was further used to calculate the adlayer thickness for specific purposes, especially biosensing. There of each functionalization. This analysis revealed an are many considerations for the advancement of im- adlayer thickness of about 8 A˚ for the 4AB func- plantable devices, including the optimization of sensi- tionalized surfaces compared to 1 A˚ for the AAP tivity and selectivity; however, the most important is adlayer, which led to the conclusion that AAP func- most likely biocompatibility. Much research has been tionalization only provided partial coverage. A stabil- dedicated to tailoring inorganic surfaces with organic ity study using 4AB-functionalized surfaces showed molecules so as to minimize toxicity. good stability in saline solutions with varying con- One of the most common classes of materials used centrations of hydrogen peroxide. Finally, inductively in biosensing is semiconductors. Indeed, silicon has been experimented with extensively for biosensors Contract grant sponsor: NSF; Contract grant number: CHE- due to its broad physical and electrical characteri- 1052809. zation (Cui et al., 2001; Kim et al., 2007). In addition, Address for reprints: Albena Ivanisevic, Department of Materials III-V semiconductors have also been used in sensing Science and Engineering, North Carolina State University, Joint De- applications. Gallium arsenide is the most commonly partment of Biomedical Engineering NCSU/UNC-CH, 911 Partner’s used III-V semiconductor, again owing to its mature Way, Raleigh, NC 27695 development (Lee et al., 2008). Gallium phosphide E-mail: [email protected] (GaP) is a III-V semiconductor that is commonly used Received 31 October 2011; Accepted with revision 20 December in optoelectronic and high-temperature devices. It is 2011 an excellent candidate for future biosensor devices because its unmodified surface has shown favorable DOI 10.1002/sca.21012 et al. Published online 9 April 2012 in Wiley Online Library (wiley biocompatibility in previous studies (Hallstrom , onlinelibrary.com) 2007). D. Ricahards et al.: Azide functionalization on GaP 333 Like many semiconductors, GaP is susceptible to molecule. AAP was chosen because of the antibio- corrosion and degradation. This process can lead to fouling capabilities poly(ethylene glycol) (PEG) like the leaching of toxic surface ions into the surround- molecules can confer on a surface. ing solution. Obviously, this poses a problem when Photochemistry on a GaP surface is not uncom- it comes to materials such as GaP being used in im- mon. In our past study, GaP was reacted with a termi- plantable devices. Therefore, our group has used or- nal alkene under UV light resulting in the formation ganic adsorbates to covalently bind to the surface of of a dense adlayer (Ivanisevic et al., 2008). To our GaP to reduce the amount of the toxic and leach- knowledge, functionalization with azides on a GaP able oxide layer (Flores-Perez et al., 2008; Richards surface has not been studied before. et al., 2010). In this article, we propose a new method Besides XPS, other surface sensitive techniques in- to covalently functionalize a GaP surface using azide cluding atomic force microscopy (AFM) and water chemistry. Azides, which are commonly used in click contact angle were used to analyze the functionalized chemistry, have the ability to form a highly reac- surfaces. A stability study was performed using water, tive nitrene intermediate when irradiated with ultra- various pH solutions, and saline with varying concen- violet (UV) light (Ziani-Cherif et al., ’99). Azides trations of hydrogen peroxide (H2O2). Finally induc- have been used to link various organic molecules to tively coupled plasma mass spectrometry (ICP-MS) polystyrene (Nahar et al., 2001), poly(ethylene tereph- was used to quantify the amount of gallium leach- thalate) (Ziani-Cherif, Imachi et al.,’99),glasscover ing from the functionalized surfaces in the stability slips (Chen et al., 2009), quartz (Harmer, ’91), and solutions. silicon (Harmer, ’91). Due to its successful functionalization of a broad range of materials, it is proposed that azides can Materials and Methods provide a new stable route to covalently link or- ganic molecules and biomolecules to a GaP surface. Surface Cleaning and Functionalization We used 4-azidophenacyl bromide (4AB) and O- (2-aminoethyl)-O-(2-azidoethyl)pentaethylene gly- GaP (100) wafers were purchased from Univer- col (AAP) as the azide reactants (see Fig. 1). Specif- sity Wafer (South Boston, MA). The (100) plane ically, 4AB was used to allow for easy identification of GaP was chosen because of its ability to ac- of its presence on a GaP surface. Its bromine group commodate a wide range of adlayers including UV- generates a distinct peak in an x-ray photoelectron reactive adlayers (Ivanisevic et al., 2008; Richards spectroscopy (XPS) spectrum that provides evidence et al., 2010). Compounds 4AB and AAP were pur- that the molecule is indeed attached to the GaP sur- chased from Sigma-Aldrich (St. Louis, MO). The face. In addition, the bromine group is not capable GaP wafers were cut into 5 mm × 5 mm pieces. Be- of bonding to both the GaP surface and the azide fore functionalization, all wafers were degreased. The Fig 1. The functionalization schemes of the two different azide adsorbates. 334 SCANNING VOL. 34, 5 (2012) degreasing process consisted of ultrasonication in wa- chased from Asylum Research (Santa Barbara, CA). ter for 20 min and ethanol for 20 min followed by dry- Data analysis was performed using the Nanoscope ing with N2. The wafers were then exposed to NH4OH III 5.12r3 software (Veeco, Plainview, NY). for 30 s in order to remove the oxide layer (Morota XPS data were obtained with a Kratos Ultra DLD and Adachi, 2006) The wafers were then rinsed with spectrometer (Shimadzu, Nakagyo-ku, Kyoto, Japan) water and ethanol and dried with N2. Finally, the using monochromatic Al Kα radiation (hν = 1486.6 wafers were exposed to either a 1-mM solution of eV). Survey and high-resolution spectra were col- 4AB in toluene or a 1% solution of AAP in ethanol. lected at a fixed analyzer pass energy of 160 and 20 eV, The reaction vials were exposed to UV light (302 nm) respectively, and acquisition was performed at pho- for 3 h. After the functionalization was complete, the toemission angles of 0◦,45◦, and 60◦ measured with wafers that were exposed to 4AB were rinsed with respect to the surface normal. Binding energy values copious amounts of toluene, water, and ethanol and were referenced to the Fermi edge, and charge cor- dried with N2 after each rinse. The wafers exposed to rection was done using the Carbon 1s peak set at AAP were rinsed with copious water and ethanol with 284.80 eV. Curve fitting was performed after linear subsequent N2 drying after each rinse. All wafers were or Shirley-type background subtraction assuming a stored in a vacuum desiccator for an average of four Gaussian/Lorentzian peak shape. days until subsequent analysis unless otherwise noted. ICP-MS data were obtained with an Element2 ICP mass spectrometer (ThermoFisher, Bremen, Ger- many) equipped with an Aridus desolvating introduc- Stability Experiments tion system (with a T1Hnebulizer) to enhance sen- sitivity and reduce oxide and hydride interferences One milliliter solutions of water; saline; saline with (Cetac Technologies, Omaha, NE).