Surface-enhanced vibrational investigation of adsorbed analgesics Merritt Smith a , Kaitlyn Stambaugh a , Lauren Smith a , Hye-Jin Son a , Amanda Gardner a , Scott Cordova a , Krystal Posey a , Donald Perry a, *, Alexandru S. Biris b a Department of Chemistry, University of Central Arkansas, Conway, AR 72035,United States b Department of Applied Sciences, University of Arkansas, Little Rock, AR 72204, United States 1. Introduction The persistence of various medicinal compounds such as ibuprofen, acetylsalicylic acid (AA) and acetaminophen in aquatic environments including drinking water supplies has been receiv- ing international attention [1,2]. Characterization of the adsorption properties of these drugs on metal surfaces both for bio-medical as well as environmental applications, will aid in the understanding of the interactions and decomposition of such analgesics in the environment. Given the formation of atomic thin monolayers of such chemicals onto various surfaces, surface-enhanced vibra- tional spectroscopies (SEVS), including surface-enhanced Raman spectroscopy (SERS) and surface-enhanced infrared absorption (SEIRA), prove to be some of the few and most powerful analytical techniques for the study of organic nanofilms that can be used to study their adsorption properties [3–6]. In recent years, the SEVS have also seen extensive use for the characterization of pharma- ceuticals and other bio-molecules. SERS is perhaps the most widely known SEVS technique used to investigate biochemical systems [7–17]. However, SEIRA has been increasingly applied in a variety of biochemical investigations [18–24]. In addition to environ- mental and biochemical applications, implementation of SEVS in the characterization of analgesic adsorption on metallic substrates will be critical in future nanotechnology applications that include drug delivery, bio-active surfaces, bio and chemical nanosensors, etc. Moreover, understanding how these molecules interact with various surfaces could provide advanced control of the biocompat- ibility of various bio-medical implants. Significant work has recently has been presented in the specialized literature combining both SERS and SEIRA in order to investigate a specific system [25–34]. Surface-enhanced vibrational spectroscopies are thought to be inherently the most sensitive to the first adsorbed layer [24]. SERS is a short-range effect whereby most of the enhancement of vibrational modes is in the monolayer [5]. On the other hand, SEIRA is a longer-range effect that can often lead to vibrational enhancement of a number of adsorbed layers [35]. In combination, these two spectroscopies have the potential to probe a multilayer film and separate the Vibrational Spectroscopy 49 (2009) 288–297 ARTICLE INFO Article history: Received 27 June 2008 Received in revised form 31 October 2008 Accepted 6 November 2008 Available online 18 November 2008 Keywords: SERS SEIRA Acetylsalicylic acid Aspirin Acetaminophen Ibuprofen ABSTRACT Adsorption properties of acetylsalicylic acid (AA), ibuprofen and acetaminophen deposited from volatile solvents with varying protic/aprotic properties on vacuum-evaporated silver films were characterized using surface-enhanced infrared absorption (SEIRA) and surface-enhanced Raman spectroscopy (SERS). SERS preferentially enhances monolayer Raman shifts, while SEIRA can enhance the infrared absorbance of the monolayer and multilayers. To our best knowledge, this is the first reported study of these molecules using a combination of SERS/SEIRA. SERS revealed that AA and ibuprofen adsorbed ionically in monolayers, independent of the deposition solvents used in the process. SEIRA experiments showed that AA multilayers condensed molecularly using a deposition solvent with polar bonds. However, when an alkane deposition solvent with non-polar bonds such as n-heptane was used, AA adsorbed as acetylsalicylate ions in the first few multilayers, while ibuprofen always adsorbed as the free acid in the multilayer. These ionization trends depend upon the affinity of AA and ibuprofen for the underlying silver film. TPD experiments on silver powders further demonstrated that ibuprofen affinity for silver was less than AA. Furthermore, SEIRA indicated that acetaminophen adsorbed as multilayers of metastable polymorphs using protic or polar aprotic deposition solvents. Protic deposition solvents gave higher quality SERS spectra of an acetaminophen monolayer in comparison to polar aprotic deposition solvents. Such studies could find significant applications in biochemical and nanotechnology processes such as drug delivery, catalysis, and tissue engineering and will contribute to the understanding of the impact and fate of analgesics released into the environment. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +1 501 450 5937; fax: +1 501 450 3623. E-mail address: donp@uca.edu (D. Perry). Contents lists available at ScienceDirect Vibrational Spectroscopy journal homepage: www.elsevier.com/locate/vibspec 0924-2031/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2008.11.002