Characterization of DSPA films onto solid substrates using optical and electrochemical techniques

With the intention to obtain biologically active substrates, different techniques have
been employed to modify solid surfaces (conductive materials, polymers, etc.) for
being applied in biosensors. For this purpose, physical and chemical immobilization
methods have been developed since the last decades of the 20th century.
Particularly, Langmuir-Blodgett deposition has the advantage to obtain controlled
accurate lipidic films stabilized by film-substrate physical interactions. In this work,
distearoylphosphatidic acid (DSPA) was used to modify different solid substrates
such as ITO (Indium Tin-Oxide) and silicon wafer (SW) by Langmuir-Blodgett
technique. DSPA films were deposited at 40 mN.m-1 with a resulting transfer rate of
0.97 onto the solid substrates. Then, DSPA layers were characterized using contact
angle, ellipsometric spectroscopy, AFM, and different electrochemical methods
(Ciclyc Voltammetry, CV and Electrochemical Impedance Spectroscopy, EIS). The
electrochemical experiments were carried out in a 50 mmol.L-1 phosphate buffer
solution containing 5 mmol.L-1 K3Fe(CN)6
/K4Fe(CN)6 at pH = 7.6.
The electrochemical data demonstrated that homogeneous compact
Langmuir-Blodgett (LB) films were obtained with DSPA onto ITO electrodes.
Accordingly, an increased resistance to the charge transfer process and a
decreasing interfacial capacitive behavior were observed by EIS. In addition, CV
experiments also showed that the electrochemical response of ITO displayed a
much more quasi-reversible behavior when the DSPA film blocked the electrode
surface. The surface modification was also evaluated by contact angle. In this case,
a hydrophobic response was developed on the hydrophilic substrate surface (55°)
upon DSPA film deposition (79°).
The experimental results demonstrated that DSPA was able to acquire stable LB
films onto hydrophilic substrates. Also, the electrochemical techniques were a helpful
tool to study DSPA layers adsorbed onto conducting materials. In this regard, DSPA
films are able to be considered as a stable platform to immobilize biological
molecules like proteins (enzymes, SpA, antibodies) for being applied in the
development of biosensors.