Layer Protein Films: Quartz Crystal Resonator Frequency and Admittance Analysis

EJ. CALVO; C DANILOWICZ; E FORZANI; A WOLOSIUK; MARCELO JAVIER OTERO

Biomolecular films: Design, function and applications

Marcel Dekker Inc.

Lugar: New York-Basel; Año: 2003; p. 337 - 380

In this chapter we describe recent advances in the use of quartz crystal microbalance (QCM) frequency shift and quartz crystal admittance/impedance analysis of self-assembled protein films complemented by ellipsometric thickness, and comparison of acoustic and optical mass/thickness. These protein films can be regarded as model systems among other applications of electrochemical biosensors based on molecular recognition andelectrical signal generation through electrocatalysis of ??wired?? (electrically connected) enzyme electrodes. Organized multilayers formed by stepwise alternated electrostatic adsorption of anionic and cationic polyelectrolytes provide a simple way to fabricate ultrathin functional ¢lms on solid surfaces with nanometer reso-lution [1]. Self-assembled protein ¢lms on electrode surfaces have the advantage over hydrogels of the same components, obtained either by random electropolymerization or sol-gel transformation, of better control of the molecular orientation and organization in the nanoscale. The method of layer-by-layer (LBL) electrostatic adsorption between a charged surface and oppositely charged molecules in solution with surface charge reversal brings the possibility of:       1.   Adsorption regulation and restriction to monolayer by repulsion of soluble molecules of equal charge2.   Adsorption of oppositely charged soluble molecules on top of thesurface layerIn order to characterize the resulting organized multilayer ¢lms, it isvery convenient to have at hand in situ methods to determine the ¢lm structure and dynamics. Among these methods, acoustic and optical techniques have been extensively used in recent years. The quartz crystal microbalance based on the thickness shear-mode wave resonator provides gravimetric information and reveals volume and viscoelastic changes. Optical techniques such as ellipsometry, surface plasmon resonance (SPR), and scanning angle re£ectometry (SAR) complement acoustic methods, and film thickness and refractive index values can be determined. Both acoustic and optical techniques result in di¡erent values of protein mass or thickness since they sense in di¡erent ways the interactions among biomolecules, polymers, and water.