Chemical modification of scanning tunneling microscopy tips for identification of functional groups in self-assembled monolayers

Since 30 years, scanning tunneling microscopy (STM) allows individual atoms or molecules to be observed. Amongst others, it is particularly well adapted to image molecular organizations at liquid/solid interfaces as it is already successful in imaging physisorbed self-assembled monolayer (SAM) structures adsorbed on graphite, highlighting a variety of physical, chemical, dynamical and electronic phenonema. In this context, another important aspect of the molecular layers investigation with STM is its ability to discriminate functional groups from the molecule backbone. However, while functional groups such as amines or thiols can be easily distinguished from the molecule skeleton in STM images, others (ester or carboxyl groups for example) can not. During the last decade, several research groups focused on the STM tip chemical modification in order to change contrast of functional groups in STM images. In this minireview, we describe the STM tip modification phenomenon and its effect on STM tunneling current. The different STM tip modification performed are listed and their application to the identification of various functional groups within SAM at the liquid/graphite interface detailed

Etude de matériaux organiques et de biomatériaux par microscopie à effet tunnel à balayage (STM) et microscopie à force atomique (AFM)

Cette thèse résume les connaissances acquises au cours de quatre années de recherches interdisciplinaires centrées sur des études de composés organiques et de biomatériaux, en vue d'applications bio- et nano-technologiques. Les systèmes sélectionnés ont ainsi révélé leurs secrets, grâce à l'utilisation des microscopies à effet tunnel à balayage (S.T.M.) et à force atomique (A.F.M.). Dans une première partie, nous développons les bases théoriques et expérimentales nécessaires à la bonne réalisation et interprétation d'images S.T.M. et A.F.M. Les différents modes d'utilisation y sont détaillés avec précision. La seconde partie présente les résultats obtenus sur différents systèmes. Celle-ci est divisée en quatre chapitres. Le premier est relatif aux monocouches auto-assemblées. Ces dépôts moléculaires fortement organisés peuvent être constitués de molécules physisorbées ou chimisorbées. Toutes deux ont été soumises à investigation. Dans le cadre de couches physisorbées, l'utilisation de la technique totalement révolutionnaire des pointes S.T.M. modifiées chimiquement a permis l'identification de la position et de l'orientation de groupements fonctionnels au sein des couches moléculaires. Nous nous sommes ensuite attardé sur l'influence du groupement lié au substrat pour des molécules chimisorbées. L'étude de matériaux organiques ne pouvait se faire sans parler d'A.D.N. Son importance fondamentale, ainsi que l'espoir énorme qu'engendre sa manipulation ont attisé notre curiosité.En particulier, nous nous sommes focalisé sur la caractérisation de transporteurs de gènes au coeur des cellules, dans notre cas, des condensats réalisés à partir d'A.D.N. et de polymère. Ensuite, nous nous sommes tourné vers les biomatériaux. Un chapitre entier traite en effet des implants métalliques. La croissance d'apatite en surface de ces implants, réalisés en oxyde de tantale, surmontés de différentes couches intermédiaires, a été analysée. L'influence des propriétés des couches intermédiaires sur la vitesse de croissance de l'apatite a plus spécifiquement retenu notre attention. Les derniers composés étudiés ont été sélectionnés pour leur utilisation potentielle au sein de systèmes biotechnologiques (capteurs solaires et/ou biologiques). Parmi ceux-ci figurent des couches de porphyrines (PPIX, ZnPPIX et TPD) sur des substrats métalliques, pouvant représenter les prémices de capteurs solaires. Des capteurs biologiques peuvent actuellement également être obtenus par le dépôt de protéines (la protéine fluorescente verte, en particulier) sur des substrats métalliques. La conservation de leurs propriétés après adsorption restant difficile à vérifier sans avoir d'informations complémentaires sur leur assemblage, nous nous y sommes intéressés. Pour terminer, nous montrerons, par spectroscopie de force, une mesure de l'interaction entre deux biomolécules, à savoir la biotine et l'avidine.(DOCSC01)--FUNDP, 200

Special features in self-assembled monolayers

Some functional groups in self-assembled monolayers have recently been identified in STM images, due to the chemical modification of gold tips [1]. In those studies, different molecules presenting different functional groups were investigated, resolving several molecular organizations [1-2, 4]. In this paper, we aimed at demonstrating that such chemically modified STM tips can be used to investigate phenomenon routinely observed with standard Pt/Ir tips, such as domain boundary dynamics in hexadecanol SAMs. It also allows transitory features to be identified. Moreover, rarely observed phenomena are also presented below, i.e. bilayers composed of wax esters molecules physisorbed at the phenyloctane/graphite interface

Biosensors - Emerging Materials and Applications

A biosensor is a detecting device that combines a transducer with a biologically sensitive and selective component. Biosensors can measure compounds present in the environment, chemical processes, food and human body at low cost if compared with traditional analytical techniques. This book covers a wide range of aspects and issues related to biosensor technology, bringing together researchers from 19 different countries. The book consists of 27 chapters written by 106 authors and divided in three sections: Biosensors Technology and Materials, Biosensors for Health and Biosensors for Environment and Biosecurity

Electrochemical and spectroscopic study of C12H25X molecules adsorption on copper sheets, X (-SH, -S-S-, -SeH and -Se-Se-)

In this contribution, we explored the possibility of using selenol and selenide molecules to form self-assembled monolayers (SAMs) on copper, in order to check the influence of anchoring groups on SAMs quality and compared it to well-known thiolate assemblies (formed with thiol and disulfide molecules). Precisely, monolayers of pure alkane chains have been self-assembled on electroreduced bulk copper. The different selected molecules present the following reactive anchoring groups: thiol (R-SH), disulfide (R-S-S-R), selenol (R-SeH) and diselenide (R-Se-Se-R), where R = C12H25-. Electrochemical (cyclic voltammetry and scanning electrochemical microscopy) techniques and spectroscopic (X-ray photoelectron and polarization modulation infrared reflection absorption spectroscopy) have been used to characterize the surface composition and monolayer organization. Atomic force microscopy (AFM) measurements complete this study. All molecules analyzed have been shown to form monolayers of variable quality. The R-SH and R-SeH monolayers seem to lead to better organized and insulating layers than the R-S-S-R and R-Se-Se-R monolayers. However, the case of the diselenide is more complex and could lead to some interesting properties. (C) 2009 Elsevier Ltd. All rights reserved

Molecular conformation and electronic properties of protoporphyrin-IX self-assembled monolayers adsorbed on Pt(111) surface

Monolayers of protoporphyrin-IX molecules are prepared on a Pt(111) surface by a self-assembly process in order to manufacture organic devices with controlled electronic properties. Scanning tunnelling microscopy (STM) and two-colour sum-frequency generation (2C-SFG) are performed ex situ in ambient air, in order to characterize their molecular conformation and electronic properties at the monolayer level, respectively. STM measurements performed with functionalized gold tips reveal a high covering rate of the metal surface. 2C-SFG measurements highlight CH stretching modes of vinyl substituted groups (R-CH=CH2) in the 2800–3200 cm-1 infrared spectral range and particular electronic features in the visible spectral range, i.e. a Soret band red shift and band separation compared to the liquid phase. Moreover, similar measurements are performed on Zn(II)Protoporphyrin-IX and 5-[p-(6-mercaptohexoxy)-phenyl]-10,15,20-triphenylporphin films for comparison. These results suggest a film conformation with the molecules having different tilt angles with respect to the substrate normal, depending on the ion metal presence or the chain length bonded to the porphyrin moiety

On functional groups in self-assembled monolayers

The influence of the tip modification on the contrast of a Scanning Tunnelling Microscopy (STM) image is investigated. This innovative technique makes it possible to identify the molecular organisation inside self-assembled monolayers (SAMs). Moreover, the observed contrast variations, between two different tip modifications, highlight functional groups position and orientation. In situ treatment of modified tips allows to switch contrast enhancement on and off