This page describes the work carried out at Columbia University as part of postdoctoral research. We used Quartz Crystal Resonator based Instrumentation for characterizing surface and interfacial phenomena. Also conducted Neutron Reflectometry experiments. The following paragraphs highlight the main activities.
 | Self-Assembled Monolayer (SAM): "One molecule" thick layers that have many present and potential applications such as, Sensors, Transducers, Detectors, Packaging, Insulating, and many others. Surface modification via blended SAM adsorption. Non-specific adsorption of polymers for micropatterning. See Applications. |
| Liquid Phase QCR/QCM: Liquid phase measurements via quartz crystal microbalance and flowcell apparatus with hydrodynamic control. |
Self-Assembled Monolayer (SAM)
This is an area that has received great deal of attention over the last decade since this family of materials allows for the preparation of highly ordered films that has a host of applications. Given the high mass sensitivity of the QCRs, it is being employed as a means of following monolayer formation on gold surfaces which is the most commonly employed substrate material in these experiments. The efforts in this area are being focused on the kinetics of the SAMs of various novel thiol molecules. The thickness of these SAMs and their formation kinetics can be quantified accurately by using the flow-cell apparatus that has been developed for this purpose.
Moreover, the surface chemistry of the adsorbing surface can be modified and/or controlled by treating it with appropriate SAM. The adsorption kinetics of polymers on the SAM treated surface is an exciting area of scientific and engineering investigation.
Few Applications of the SAMs and Thin-Films
1. Sensors, Transducers, Detectors and Displays: Using selective SAMs the surface chemistry of an adsorbing surface can be manipulated in a variety of way. For instance, covering a surface with methyl terminated SAMs would introduce hydrophobic characteristics while if the same surface is cover with acid terminated SAMs it will become hydrophilic. Varying the mole fraction of the hydrophilic SAM in a blend, for instance, a chemistry-controlled surface can be created. These properties can be uniquely utilized in selective sensor applications.
Piezoelectric polymers are becoming increasingly well known and utilized as sensors and transducers. The best known piezoelectric polymer is poly(vinylidene fluoride) (PVF2). It has many desirable properties, including insensitivity to water, thermal stability in its piezoelectric behavior to about 100°C, toughness and a good impedance match to water (making it a good acoustic sensor in the ocean).
The rapid decline in the cost of electronic information processing has stimulated a need to interface electronic systems with the real world. Thus, sensors and output transducers has a potential of enormous growth above and beyond what has been achieved over the past few decades.
2. Packaging and Insulating Layers
for Integrated Circuits
3. Patternability: Resists and
Information Storage
4. Electronic Circuit Components
5. Functionalization of Surfaces
6. Thin Coatings for Electrodes
7. Study of Intermolecular Forces
QCR based Apparatus for liquid phase measurements
The adsorption of flexible macromolecules onto solids causes a dramatic surface modification. Such surface modification is a very important research topic at the present time. Examples include technologies relying on colloidal suspensions where adsorption affects suspension stability and rheology, extrusion and molding operations where adsorption on machine surfaces affects flow stability and fouling, and in biomedical technologies involving prosthetic and blood contacting devices where biopolymer adsorption controls thrombosis and cell adhesion.
While significant advances have been made toward understanding the static equilibrium, the dynamic and nonequilibrium aspects of polymer adsorption remain largely unexplored. These dynamic features control the formation, response, and long-term stability of adsorbed layers, so their understanding is critical to advancing the relevant technologies.
The principal goal of this work was to design and evaluate QCR based apparatus for liquid phase measurements with hydrodynamic control. The aim of these studies is to develop a better understanding of these aspects in model systems and then apply the acquired fundamentals to systems of vital interest such as adhesion of dendrimers and DNA.
Design of the Flow-cell
Most QCR based work reported in the contemporary
literature use a static measurement cell, where the quartz crystal is mounted on a frame
by means of a pair of O-rings. The biggest limitations of the static cell is that it
applies an addition stress,
on the QCR from the mounting arrangement. While there is a way of
quantifying this stress (see later), the most desirable situation is to eliminate it.
Moreover, where liquid-phase flow is involved, a QCR based measurement system is expected
to meet the following requirements.
- The flow needs to be laminar.
- The pressure loss through the entire system must be small enough that commercially available pumps can be used.
- The pump needs to provide a steady, pulseless flow.
- Many of the fluids are expensive and difficult to prepare, therefore, the volume of the entire system including cell, tubing and pump, should be kept to a minimum.
- The cell and components should be inert and easy to clean.
- Polymer and/or viscous fluid properties are greatly influenced by temperature; therefore, a temperature controlled environment is necessary to ensure stable and reproducible measurements.
Based on the above requirements a flat-channel flow-cell for liquid-phase QCR measurements was been constructed. The cell allows flush-mounting of a QCR at a position in the channel where predictable fluid-dynamic conditions exist. Being flash mount on a precisely machined groove, the crystal can function under a completely stress-free condition, that is, no additional stress is experienced from the cell mounting arrangement. This factor is expected to have a significant impact on the sensitivity of the crystal. An oscillator circuit (acquired from Maxtek, Inc.) is placed just beneath the QCR allowing the use of short lead-length to minimize stray-capacitance and inductance. A controller (model PI-70, acquired from Maxtek, Inc.) is used to drive the oscillator circuit as well as to measure the frequency response of the QCR and a magnetic gear pump-head was used to obtain a pulseless flow of the experimental fluids.
Kinetics of SAM adsorption on gold surface
Using the setup described above, the adsorption kinetics of a Self-Assembled Monolayer (SAM) of biphenyl thiol molecules was measured. A frequency shift of 7±1 Hz is measured while the predicted shift is ~8.5 Hz.
Calculation of the frequency shift due to SAM adsorption.
QCM surface area Aq = 1.26677 cm2, Resonance frequency fs = 5×106. Area per thiolet, Ath = 20×10-16 cm2 [7,8]. Assuming uniform coverage, the number of molecules adsorbed on the surface is given by total surface area/area per molecule; therefore the adsorbed mass on the surface for a monolayer is M = 2.10656×10-7 g.Calculation of expected frequency shift:
From calibration procedure: Slopes m1 = 11602, m2 = 10589, ratio = m1/m2. From Eq. (5), a' = a(b'/b) = 2.86729×107. Finally,
Dfm = a'b(M/Aq) ~ 8.5 Hz.
It may be emphasised here that in order to arrive at the predicted frequency shift, a proper calibration of the apparatus is essential. As described in ref [2], the calibration process replaces the constants a and b by a' and b' respectively, thereby taking care of the crystal surface roughness. For instance, using the Suerbrey equation [Eq. (1)], a shift of ~9.5 Hz is predicted for the same SAM. However, high quality quartz crystals, having surface roughness ~100 nm, were used in the present experiments which accounts for rather small %disagreement (~10%). For crystals with higher surface roughness, the %disagreement between the measured and predicted value obtained from Suerbrey equation increases.
Conclusions
The flat-channel flow-cell apparatus was subjected to further evaluation. From the
calibration procedure it was found that the slopes obtained from
plot of the two setups discussed above are within ~3% of each other.
The slopes corresponding to
plot of the same
data are within ~12% of each other. However, the y-intercepts of
plot and the y-intercepts of
plot of the data obtained from the flow-cell apparatus are significantly larger compared
to those obtained from the static Kynar probe setup. This difference is assumed to have
arisen from the special design of the flow-cell apparatus. That is, under the present
design the QCR can function in a stress-free condition. However, further work is needed to
improve the stability in contact with organic solvents and to implement temperature
control.
The results of Dendrimer measurements were published in Langmuir.
© 1999 Anis Rahman
