J. Conti

Dynamics of Wetting Revisited

We present new spreading-drop data obtained over four orders of time and apply our new analysis tool G-Dyna to demonstrate the specific range over which the various models of dynamic wetting would seem to apply for our experimental system. We follow the contact angle and radius dynamics of four liquids on the smooth silica surface of silicon wafers or PET from the first milliseconds to several seconds. Analysis of the images allows us to make several hundred contact angle and droplet radius measurements with great accuracy. The G-Dyna software is then used to fit the data to the relevant theory (hydrodynamic, molecular-kinetic theory, Petrov and De Ruijter combined models, and Shikhmurzaevs formula). The distributions, correlations, and average values of the free parameters are analyzed and it is shown that for the systems studied even with very good data and a robust fitting procedure, it may be difficult to make reliable claims as to the model which best describes results for a given system. This conclusions also suggests that claims based on smaller data sets and less stringent fitting procedures should be treated with caution.

Surface functionalization of germanium ATR devices for use in FTIR-biosensors

Biosensors based on intrinsic detection methods have attracted growing interest. The use of Fourier transform infra-red (FTIR) spectroscopy with the attenuated internal total reflection (ATR) mode, in the biodetection context, requires appropriate surface functionalization of the ATR optical element. Here, we report the direct grafting of a thin organic layer (about 20 A depth) on the surface of a germanium crystal. This covering, constructed with novel amphiphilic molecules 2b (namely, 2,5,8,11,14,17,20-heptaoxadocosan-22-yl-3-(triethoxysilyl) propylcarbamate), is stable for several hours under phosphate buffered saline (PBS) flux and features protein-repulsive properties. Photografting of molecule 5 (namely, O-succinimidyl 4-(p-azidophenyl)butanoate) affords the activated ATR element, ready for the covalent fixation of receptors, penicillin recognizing proteins BlaR-CTD for instance. The different steps of the previous construction have been monitored by water contact angle (theta(w)) measurements, spectroscopic ellipsometry (covering depth), X-ray photoelectron spectroscopy (XPS) by using a fluorinated tag for the control of surface reactivity, and FTIR-ATR spectroscopy for the structural analysis of grafted molecules. Indeed, contrarily to silicon device, germanium device offers a broad spectral window (1000-4000 cm(-1)) and thus amide I and II absorption bands can be recorded. This work lays the foundations for the construction of novel FTIR biosensors.

Superhydrophobic surfaces from various polypropylenes

Superhydrophobic surfaces were prepared from solutions of isotactic polypropylenes of various molecular weights using soft chemistry. Varying the conditions of the experiments (polymer concentration and initial amount of the coated solution) allowed us to optimize the superhydrophobic behavior of the polymer film. Results show that decreasing the concentration and/or film thicknesses decreases the probability to get superhydrophobicity for all polypropylenes tested. Measurement and analysis of advancing and receding contact angles as well as estimation of surface homogeneity were performed. Similar results were obtained with syndio- as well as atactic polypropylenes.

Drop Impact on Soft Surfaces: Beyond the Static Contact Angles

The wettability of cross-linked poly(dimethylsiloxane) elastomer films and of octadecyltrichlorosilane self-assembled monolayers with water has been measured and compared using various methods. Contact angle hysteresis values were compared with values reported in the literature. A new method to characterize advancing, receding contact angles, and hysteresis using drop impact have been tested and compared with usual methods. It has been found that for the rigid surfaces the drop impact method is comparable with other methods but that for elastomer surfaces the hysteresis is function of the drop impact velocity which influences the extent of the deformation of the soft surface at the triple line.

Ligand-receptor interactions in complex media: A new type of biosensors for the detection of coagulation factor VIII

Detection of receptor-ligand interaction in complex media remains a challenging issue. We report experimental results demonstrating the specific detection of the coagulation factor VIII in the presence of a large excess of other proteins using the new BIA-ATR technology based on attenuated total reflection Fourier transform infrared spectroscopy. The principle of the detection is related to the ability of factor VIII molecules to bind to lipid membranes containing at least 8% phosphatidylserine. Several therapeutic concentrates of factor VIII were analyzed and the binding of the coagulation factor was monitored as a function of time. We show that a non-specific adsorption of stabilizing agents (typically, von Willebrand factor and human serum albumin) may be avoided by controlling the geometry of the ATR element. A linear response of the sensors as a function of the factor VIII concentration is described for different lipid membrane compositions.

Dissipation and moving contact lines on non-rigid substrates

We consider the dynamics of spreading of silicon oil droplets on elastomer thin films, and the associated relaxation of the advancing contact angle. Although the viscoelastic breaking of the droplet is observed on bulk elastomers, according to the studies of Shanahan and coworkers [C. R. Acad. Sci. Paris 317 (1993) 1153], we show that the breaking progressively disappears as the elastomer film thins.

Can we predict the spreading of a two-liquid system from the spreading of the corresponding liquid-air systems?

We present new data obtained from the spreading of a series of oil droplets, on top of a hydrophobic grafted silicon substrate, in air and immersed in water. We follow the contact angle and radius dynamics of hexane, dodecane, hexadecane, dibutyl phthalate, and squalane from the first milliseconds to approximately 1 s. Analysis of the images allows us to make several hundred contact angle and droplet radius measurements with great accuracy. The G-Dyna (Seveno et al. Langmuir 2010, 25, 13034) software is then used to fit the data with one of the wetting theories, the molecular-kinetic theory (MKT) (Blake et al. J. Colloid Interface Sci.1969, 30, 421), which takes into account the dissipation at the three-phase zone at the contact line. This theory allows us to extract the coefficient of friction of the contact line, which expresses the relationship between the driving force, that is, the unbalanced Young force, and the contact-line velocity V. It is first shown that the MKT is appropriate to describe the experimental data and then that the contact-line friction is a linear function of the viscosity as theoretically predicted. This is checked for oil-air and oil-water systems. A linear relation between the contact-line friction measured in oil-water systems and the contact-line frictions of the parent single liquid system seems plausible. To the best of our knowledge, this is the first trial to establish a link between the dynamics of wetting in liquid-liquid and in liquid-air systems.

Fourier transform infrared immunosensors for model hapten molecules

We report experimental results concerning the detection of 2,4-dinitrophenol, under its free form or coupled to human serum albumin using Fourier transform infrared spectroscopy-based sensors. Competitive immunoreactions were carried out using several anti-dinitrophenol monoclonal antibodies. Comparison with enzyme-linked immunosorbent assays in competition is given for standard operating conditions. FTIR detection limits are comparable to those obtained by ELISA. The limits of detection are about 5-15 ng/mL for the coupled DNP. Using the LO-DNP61 antibody, a detection limit of congruent with 5 ng/mL was also estimated for the free DNP molecules but is much higher for the other antibodies.

Gastric ATPase phosphorylation/dephosphorylation monitored by new FTIR-based BIA–ATR biosensors

Biosensors are composite devices suitable for the investigation of receptor-ligand interactions. In this paper we present the specific application to a membrane embedded protein of a new sensor device, so-called BIA-ATR, based on Attenuated Total Reflection-Fourier Transform Infrared (ATR-FTIR) spectroscopy. It consists in a functionalised ATR germanium crystal whose surface has been covalently modified to adsorb a biomembrane. Detection of the ligand-receptor interaction is achieved using FTIR spectroscopy. We report the specific detection of the phosphorylation/dephosphorylation of the H+/K+ gastric ATPase. The H+, K+-ATPase is a particularly large protein entity. This glycosylated protein contains more than 1300 residues and is embedded in a lipid membrane. Yet we demonstrate that the BIA-ATR sensor is capable of monitoring the binding of a single phosphate on such a large protein entity. Furthermore, we also demonstrate the potential of the approach to monitor the kinetics of binding and dissociation of the ligand.

Designing a high performance, stable spectroscopic biosensor for the binding of large and small molecules

In the context of FTIR ATR-based sensors, the organic layer covering the ATR element has to be as stable as possible for optimal spectroscopic measurements. Previously, this self-assembled covering was considered stable after several hours under a PBS flux, probably due to a hydrophobic barrier, which prevents water penetration into the grafted network. Stability and reactivity, measured simultaneously using FTIR ATR, identify the limits of the previously used molecular construction. For the first time, surface etching of the previous functionalised Ge devices (Ge-PEG-NHS), a few minutes after BSA injection, was observed. It was concluded that the molecular chain deformation of Ge-PEG-NHS likely occurred when large molecules were bound. BSA loaded onto a Ge-PEG-NHS surface led to network deprotection, with the probable disruption of hydrogen bonds for single barrier-based networks. This, in turn, was presumably influenced by the random deposition of the NHS moiety on the PEG chain. A new functionalised germanium device, using a rapid three-step in situ procedure, provides an efficient robust network composed of two protective barriers, ideal for the binding of various sized molecules. The Ge-APS-PEG-NHS device has shown exceptional sensitivity with regards to BSA and ethanolamine target molecules while offering homogeneous NHS distribution.