Sample Preparation in Quartz Crystal Microbalance Measurements of Protein Adsorption and Polymer Mechanics

Summary

The quartz crystal microbalance can provide accurate mass and viscoelastic properties for films in the micron or submicron range, which is relevant for investigations in biomedical and environmental sensing, coatings, and polymer science. The sample thickness influences which information can be obtained from the material in contact with the sensor.

Abstract

In this study, we present various examples of how thin film preparation for quartz crystal microbalance experiments informs the appropriate modeling of the data and determines which properties of the film can be quantified. The quartz crystal microbalance offers a uniquely sensitive platform for measuring fine changes in mass and/or mechanical properties of an applied film by observing the changes in mechanical resonance of a quartz crystal oscillating at high frequency. The advantages of this approach include its experimental versatility, ability to study changes in properties over a wide range of experimental time lengths, and the use of small sample sizes. We demonstrate that, based on the thickness and shear modulus of the layer deposited on the sensor, we can acquire different information from the material. Here, this concept is specifically exploited to display experimental parameters resulting in mass and viscoelastic calculations of adsorbed collagen on gold and polyelectrolyte complexes during swelling as a function of salt concentration.

Introduction

The quartz crystal microbalance (QCM) leverages the piezoelectric effect of a quartz crystal to monitor its resonant frequency, which is dependent on the mass adhered to the surface. The technique compares the resonant frequency and bandwidth of an AT cut quartz crystal sensor (typically in the range of 5 MHz)¹ in air or a fluid to the frequency and bandwidth of the sensor after deposition of a film. There are several benefits for using the QCM to study thin film properties and interfaces, including the high sensitivity to mass and potentially to viscoelastic property changes (depending on sample uniformity and thickness), the ability to perfor....

Protocol

QCM-D Collagen Adsorption

1. Sample Preparation and Sensor Pre-cleaning

1. Prepare 20 mL of 0.1 M acetate buffer, adjusting the pH with HCl and NaOH as necessary to achieve pH = 5.6.
2. Add rat tail collagen solution to the 20 mL of acetate buffer under sterile conditions to a final concentration of 10 µg/mL.
3. Clean the gold-coated quartz sensor to remove organic and biological material^25,26.
   1. Place the sensor active side up in a UV/Ozone chamber and treat the surface for approximately 10 min.
   2. Heat a 5:1:1 mixture of de....

Results

The changes in frequency with time during protein adsorption exhibit a characteristic curve and plateau shown in Figure 3A-B. The initial buffer wash of 1x PBS across the bare sensor surface induces only negligible changes in frequency, offering a steady baseline to act as a reference for future data points. The introduction of collagen solution causes protein adsorption to begin, observed as a steady decrease in frequency over time, until the density of adhered collagen pla.......

Discussion

The collagen adsorption results span the Sauerbrey and viscoelastic regimes. By plotting the frequency shifts normalized to the corresponding harmonic number, we observe that the Sauerbrey limit holds true for approximately the first 2 h of the measurement. With increasing mass adhering to the sensor, however, the normalized frequency shifts for the third and fifth harmonics begin to deviate from one another (t > 2 h), indicating an ability to determine viscoelastic properties of the adsorbed film.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Acetic acid	Sigma-Aldrich	A6283	For collagen adsorption
Ammonium hydroxide solution	Sigma-Aldrich	221228	For collagen adsorption
Aqueous QCM probe	AWSensors	CLS 00050 A	For polyelectrolyte swelling
Collagen I Rat Protein, Tail	Thermo Fisher Scientific	A1048301	For collagen adsorption
Distilled water	Sigma-Aldrich	EM3234	For polyelectrolyte swelling; generally easy to acquire in research labs, but there is a catalog number in case it is not accessible
Ethanol	Sigma-Aldrich	793175-1GA-PB	For polyelectrolyte swelling
Gibco Phosphate Buffered Saline	Thermo Fisher Scientific	20012-027	For collagen adsorption
Hellmanex III	Sigma-Aldrich	Z805939	For collagen adsorption
Hydrogen peroxide solution	Sigma-Aldrich	216763	For collagen adsorption
Kimberly-Clark Professional Kimtech Science Kimwipes Delicate Task Wipers, 1-Ply	Fisher Scientific	06-666A	For polyelectrolyte swelling
NP2K VNA	Makarov Instruments		For polyelectrolyte swelling
Poly(diallyldimethylammonium chloride), MW 200,000	Sigma-Aldrich	409022	For polyelectrolyte swelling; for full synthesis procedure see Sadman et al.
Poly(styrene-sulfonate) sodium salt 30% weight in water	Sigma-Aldrich	561967-500G	For polyelectrolyte swelling; for full synthesis procedure see Sadman et al.
Potassium Bromide	Sigma-Aldrich	793604-1KG	For polyelectrolyte swelling
QSense QCM Explorer System	Biolin Scientific		For collagen adsorption
Sodium acetate, anhydrous	Sigma-Aldrich	S2889	For collagen adsorption
Spin coater, Model WS-650MZ-23NPP	Laurell technologies		For polyelectrolyte swelling