This work was supported by the NSF (DMR-1710491, OISE-1743748). J.R. and E.S. acknowledge support from the NSF (DMR-1751308).

QCM-D Collagen Adsorption
1. Sample Preparation and Sensor Pre-cleaning
<ol>
	<li>Prepare 20 mL of 0.1 M acetate buffer, adjusting the pH with HCl and NaOH as necessary to achieve pH = 5.6.</li>
	<li>Add rat tail collagen solution to the 20 mL of acetate buffer under sterile conditions to a final concentration of 10 &#181;g/mL.</li>
	<li>Clean the gold-coated quartz sensor to remove organic and biological material25,26</su...

<ol>
	<li>Marx, K. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quartz+crystal+microbalance:+A+useful+tool+for+studying+thin+polymer+films+and+complex+biomolecular+systems+at+the+solution+-+Surface+interface.">Quartz crystal microbalance: A useful tool for studying thin polymer films and complex biomolecular systems at the solution - Surface interface.</a> Biomacromolecules. 4 (5), 1099-1120 (2003).</li><li>Kleber, C., Hilfrich, U., Schreiner, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+situ+QCM+and+TM-AFM+investigations+of+the+early+stages+of+degradation+of+silver+and+copper+surfaces.">In situ QCM and TM-AFM investigations of the early stages of degradation of silver and copper surfaces.</a> Applied Surface Science. 253 (7), 3712-3721 (2007).</li><li>Yeh, C. J., Hu, M., Shull, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Oxygen+Inhibition+of+Radical+Polymerizations+Investigated+with+the+Rheometric+Quartz+Crystal+Microbalance.">Oxygen Inhibition of Radical Polymerizations Investigated with the Rheometric Quartz Crystal Microbalance.</a> Macromolecules. 51 (15), 5511-5518 (2018).</li><li>Sturdy, L. F., Yee, A., Casadio, F., Shull, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+characterization+of+alkyd+cure+kinetics+with+the+quartz+crystal+microbalance.">Quantitative characterization of alkyd cure kinetics with the quartz crystal microbalance.</a> Polymer. 103, 387-396 (2016).</li><li>Delgado, D. E., Sturdy, L. F., Burkhart, C. W., Shull, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Validation+of+quartz+crystal+rheometry+in+the+megahertz+frequency+regime.">Validation of quartz crystal rheometry in the megahertz frequency regime.</a> Journal of Polymer Science, Part B: Polymer Physics. , 1-9 (2019).</li><li>Bilchak, C. R., Huang, Y., Benicewicz, B. C., Durning, C. J., Kumar, S. 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P., Murthy, N. S., Sommerfeld, S. D., Br&#225;s, M. M., Migonney, V., Kohn, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Competitive+Adsorption+of+Plasma+Proteins+Using+a+Quartz+Crystal+Microbalance.">Competitive Adsorption of Plasma Proteins Using a Quartz Crystal Microbalance.</a> ACS Applied Materials and Interfaces. 8 (21), 13207-13217 (2016).</li><li>Keller, C. A., Kasemo, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Surface+specific+kinetics+of+lipid+vesicle+adsorption+measured+with+a+quartz+crystal+microbalance.">Surface specific kinetics of lipid vesicle adsorption measured with a quartz crystal microbalance.</a> Biophysical Journal. 75, 1397-1402 (1998).</li><li>Olsson, A. L. J., Quevedo, I. R., He, D., Basnet, M., Tufenkji, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Using+the+quartz+crystal+microbalance+with+dissipation+monitoring+to+evaluate+the+size+of+nanoparticles+deposited+on+surfaces.">Using the quartz crystal microbalance with dissipation monitoring to evaluate the size of nanoparticles deposited on surfaces.</a> ACS Nano. 7 (9), 7833-7843 (2013).</li><li>Xu, X., Zhang, C., Zhou, Y., Cheng, Q. L. J., Yao, K., Chen, Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quartz+crystal+microbalance+study+of+protein+adsorption+on+chitosan,+chitosan/poly(vinyl+pyrrolidone)+blends+and+chitosan-graft-poly(vinyl+pyrrolidone)+surfaces.">Quartz crystal microbalance study of protein adsorption on chitosan, chitosan/poly(vinyl pyrrolidone) blends and chitosan-graft-poly(vinyl pyrrolidone) surfaces.</a> Journal of Bioactive and Compatible Polymers. 22, 195-206 (2007).</li><li>Weber, N., Pesnell, A., Bolikal, D., Zeltinger, J., Kohn, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Viscoelastic+properties+of+fibrinogen+adsorbed+to+the+surface+of+biomaterials+used+in+blood-contacting+medical+devices.">Viscoelastic properties of fibrinogen adsorbed to the surface of biomaterials used in blood-contacting medical devices.</a> Langmuir. 23, 3298-3304 (2007).</li><li>Johannsmann, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Viscoelastic,+mechanical,+and+dielectric+measurements+on+complex+samples+with+the+quartz+crystal+microbalance.">Viscoelastic, mechanical, and dielectric measurements on complex samples with the quartz crystal microbalance.</a> Physical Chemistry Chemical Physics. 10 (31), 4516-4534 (2008).</li><li>Denolf, G. C., Sturdy, L. F., Shull, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-frequency+rheological+characterization+of+homogeneous+polymer+films+with+the+quartz+crystal+microbalance.">High-frequency rheological characterization of homogeneous polymer films with the quartz crystal microbalance.</a> Langmuir. 30 (32), 9731-9740 (2014).</li><li>Martin, E. J., Mathew, M. T., Shull, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Viscoelastic+properties+of+electrochemically+deposited+protein/metal+complexes.">Viscoelastic properties of electrochemically deposited protein/metal complexes.</a> Langmuir. 31 (13), 4008-4017 (2015).</li><li>Sturdy, L., Casadio, F., Kokkori, M., Muir, K., Shull, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quartz+crystal+rheometry:+A+quantitative+technique+for+studying+curing+and+aging+in+artists'+paints.">Quartz crystal rheometry: A quantitative technique for studying curing and aging in artists' paints.</a> Polymer Degradation and Stability. 107, 348-355 (2014).</li><li>Sadman, K., Wiener, C. G., Weiss, R. A., White, C. C., Shull, K. R., Vogt, B. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+Rheometry+of+Thin+Soft+Materials+Using+the+Quartz+Crystal+Microbalance+with+Dissipation.">Quantitative Rheometry of Thin Soft Materials Using the Quartz Crystal Microbalance with Dissipation.</a> Analytical Chemistry. 90 (6), 4079-4088 (2018).</li><li>Wasilewski, T., Szulczy&#324;ski, B., Kamysz, W., G&#281;bicki, J., Namie&#347;nik, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evaluation+of+three+peptide+immobilization+techniques+on+a+qcm+surface+related+to+acetaldehyde+responses+in+the+gas+phase.">Evaluation of three peptide immobilization techniques on a qcm surface related to acetaldehyde responses in the gas phase.</a> Sensors (Switzerland). 18 (11), 1-15 (2018).</li><li>Lvov, Y., Ariga, K., Kunitake, T., Ichinose, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Assembly+of+Multicomponent+Protein+Films+by+Means+of+Electrostatic+Layer-by-Layer+Adsorption.">Assembly of Multicomponent Protein Films by Means of Electrostatic Layer-by-Layer Adsorption.</a> Journal of the American Chemical Society. 117 (22), 6117-6123 (1995).</li><li>Sauerbrey, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Verwendung+von+Schwingquarzen+zur+W&#228;gung+d&#252;nner+Schichten+und+zur+Mikrow&#228;gung.">Verwendung von Schwingquarzen zur W&#228;gung d&#252;nner Schichten und zur Mikrow&#228;gung.</a> Zeitschrift f&#252;r Physik. 155 (2), 206-222 (1959).</li><li>Sadman, K., Wang, Q., Chen, Y., Keshavarz, B., Jiang, Z., Shull, K. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Influence+of+Hydrophobicity+on+Polyelectrolyte+Complexation.">Influence of Hydrophobicity on Polyelectrolyte Complexation.</a> Macromolecules. 50 (23), 9417-9426 (2017).</li><li>Sievers, P., Mo&#223;, C., Schr&#246;der, U., Johannsmann, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Use+of+torsional+resonators+to+monitor+electroactive+biofilms.">Use of torsional resonators to monitor electroactive biofilms.</a> Biosensors and Bioelectronics. 110, 225-232 (2018).</li><li>Ringberg, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Q-Sense+Explorer+Operator+Manual.">Q-Sense Explorer Operator Manual.</a> Biolin Scientific. , (2017).</li><li>Ringberg, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Q-Sense+User+Guide:+Instrument+care+and+sensor+pre-cleaning.">Q-Sense User Guide: Instrument care and sensor pre-cleaning.</a> Biolin Scientific. , (2015).</li><li>Kern, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Evolution+of+Silicon+Wafer+Cleaning+Technology.">The Evolution of Silicon Wafer Cleaning Technology.</a> Journal of The Electrochemical Society. 137 (6), 1887 (1990).</li><li>Sadman, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> sadmankazi/QCM-D-Analysis-GUI: QCMD-Analyze. , (2018).</li></ol>

The authors have nothing to disclose.

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 &#62; 2 h), indicating an ability to determine viscoelastic properties of the adsorbed film.
<p cl...

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 perform studies in situ2, and the ability to probe a much shorter rheological timescale than traditional shear rheology or dynamic mechanical analysis (DMA). Probing a short rheological timescale allows observation of how the response at this timescale changes both over extremely short (ms)3 and long (years) durations4. This capability is beneficial for the study of a variety of kinetic processes and is also a useful extension of traditional rheometric techniques5,6.
The high sensitivity of the QCM has also led to its heavy use in biological applications studying the fundamental interactions of extremely small biomolecules. An uncoated or functionalized sensor surface can be used to investigate protein adsorption; even further, biosensing through complex binding events between enzymes, antibodies, and aptamers can be examined based on changes in mass7,8,9. For instance, the technique has been used to understand the transformation of vesicles to a planar lipid bilayer as a two-phase process of adsorption of fluid-containing vesicles to a rigid structure by observing correlating changes in frequency and viscoelasticity10. In recent years, the QCM has additionally offered a robust platform to monitor drug delivery by vesicles or nanoparticles11. At the intersection of materials engineering and molecular and cellular biology, we can use the QCM to elucidate key interactions between materials and bioactive components like proteins, nucleic acids, liposomes, and cells. For example, protein adsorption to a biomaterial mediates downstream cellular responses such as inflammation and is often used as a positive indicator of biocompatibility, while in other instances extracellular protein attachment to coatings that interface with blood could induce dangerous clotting in vessels12,13. The QCM can therefore be used as a tool to select candidates optimal for different needs.
Two common approaches for performing QCM experiments collect analogous data from the experiment: the first approach records the frequency shift and the half bandwidth (&#915;) of the conductance peak. The second approach, QCM with dissipation (QCM-D), records the frequency shift and the dissipation factor, which is directly proportional to &#915; through equation 1,14
<img alt="Equation 1" src="/files/ftp_upload/60584/60584eq1.jpg" /> (1)
where D is the dissipation factor and &#402; is the frequency. Both D and &#915; are related to the damping effect the film has on the sensor, which gives an indication of the stiffness of the film. The subscript n denotes the frequency overtone or harmonic, which are the odd resonant frequencies of the quartz sensor (n = 1, 3, 5, 7&#8230;). Further discussion of models using multiple harmonics to obtain the mass and viscoelastic properties of a film can be found in a review by Johannsmann14 and previous papers from the Shull group15,16,17,18.
One key consideration for preparing QCM samples is how to apply the thin film on the sensor surface. Some common methods include spin coating, dip coating, drop coating, or adsorption of the film onto the sensor surface during the experiment19,20. There are four regions for QCM samples: the Sauerbrey limit, the viscoelastic regime, the bulk regime, and the overdamped regime. For sufficiently thin films, the Sauerbrey limit applies, where the frequency shift (&#916;&#402;) provides the surface mass density of the film. Within the Sauerbrey limit, the frequency shift scales linearly with the resonant harmonic, n, and changes in damping factor (D or &#915;) are generally small. In this regime sufficient information is not available to uniquely determine the rheological properties of the layer without making additional assumptions. Data in this regime are used to calculate the surface mass density (or thickness if the density is known a priori) of the film. In the bulk regime where the medium in contact with the crystal is sufficiently thick, the evanescent shear wave propagates into the medium before being completely dampened. Here, no mass information can be obtained using &#916;&#402;. However, in this region, the viscoelastic properties are reliably determined using the combination of &#916;&#402; and &#916;&#915; 15,18. In the bulk regime, if the medium is too rigid, the film will damp out the resonance of the sensor, preventing the collection of any reliable data from the QCM. The viscoelastic regime is the intermediate regime where the film is thin enough to have the shear wave fully propagate through the film as well as have reliable values for the damping factor. The damping factor and &#916;&#402; can then be used to determine the viscoelastic properties of the film as well as its mass. Here, the viscoelastic properties are given by the product of the density and the magnitude of the complex shear modulus |G*|p and the phase angle given by &#934; = arctan(G&#34; / G&#39;). When films are prepared in the Sauerbrey limit, the mass per unit area can be directly calculated based on the Sauerbrey equation shown below21,
<img alt="Equation 2" src="/files/ftp_upload/60584/60584eq2.jpg" /> (2)
where &#916;&#402;n is the change in the resonant frequency, n is the overtone of interest, &#402;1 is the resonant frequency of the sensor, &#916;m / A is the mass per area of the film, and Zq is the acoustic impedance of quartz, which for AT cut quartz is Zq = 8.84 x 106 kg / m2s. The viscoelastic regime is most appropriate for the study of polymer films, and the bulk limit is useful for studying viscous polymer22 or protein solutions16. The different regimes depend on the properties of the material of interest, with the optimum thickness for full viscoelastic and mass characterization generally increasing with the film stiffness. Figure 1 describes the four regions with respect to the areal density of the film, complex shear modulus, and phase angle, where we have assumed a specific relationship between the phase angle and the film stiffness that has been shown to be relevant to materials of this type. Many films of practical interest are too thick for studying the viscoelastic properties with QCM, such as certain biofilms, where the thicknesses are on the order of tens to hundreds of microns23. Such thick films are generally not appropriate for studying using the QCM, but may be measured using much lower frequency resonators (such as torsional resonators)23, allowing the shear wave to propagate further into the film.
To determine which regime is relevant for a given QCM sample, it is important to understand the d / &#955;n parameter, which is the ratio of the film thickness (d) to the shear wavelength of the mechanical oscillation of the quartz crystal sensor (&#955;n)15,16,18. The ideal viscoelastic regime is d / &#955;n = 0.05 - 0.218, where values below 0.05 are within the Sauerbrey limit and values above 0.2 approach the bulk regime. A more rigorous description of d / &#955;n is provided elsewhere15,18, but it is a quantitative parameter delineating the Sauerbrey limit and the viscoelastic limit. The analysis programs used below provide this parameter directly.
There are some additional limitations to analyzing thin films with the QCM. The Sauerbrey and viscoelastic calculations assume the film is homogeneous both throughout the film thickness and laterally across the electrode surface of the QCM. While this assumption makes it challenging to study films which have voids or fillers present, there have been some QCM investigations into films consisting of grafted nanoparticles6. If the heterogeneities are small compared to the overall film thickness, reliable viscoelastic properties of the composite system can still be obtained. For more heterogeneous systems, values obtained from a viscoelastic analysis should always be viewed with great caution. Ideally, results obtained from systems with unknown heterogeneity should be validated against systems which are known to be homogeneous. This is the approach we have taken in the example system described in this paper.
An important point that we illustrate in this paper is the exact correspondence between QCM measurements done in the frequency domain (where &#915; is reported) and the time domain experiments (where D is reported). Results from two different QCM experiments, one time domain and one frequency domain, are described, each involving a different but conceptually related model system. The first system is a simple example of collagen attachment to the sensor to illustrate representative binding kinetics and equilibration of adsorption over time during a time domain (QCM-D) measurement. Collagen is the most abundant protein in the body, known for its versatility of binding behaviors and morphology. The collagen solution used here does not require additional functionalization of the sensor&#39;s gold surface to induce adsorption9. The second experimental system is a polyelectrolyte complex (PEC) composed of anionic polystyrene sulfonate (PSS) and cationic poly(diallyldimethylammonium) (PDADMA) prepared in the same fashion as Sadman et al.22. These materials swell and become soft in salt (KBr in this case) solutions, offering a simple platform for studying polymer mechanics using a frequency domain approach (QCM-Z). For each protocol, the process of preparing, taking, and analyzing a measurement is shown in Figure 2. The schematic shows that the main difference between the QCM-Z and QCM-D approaches is in the data collection step and the instrumentation used in the experiment. All the mentioned sample preparation techniques are compatible with both approaches, and each approach can analyze samples in the three regions depicted in Figure 1.
Our data demonstrate that the preparation of samples, whether by sensor coating before or during a measurement, dictates the ability to extract the viscoelastic properties of a system. By designing the early stages of an experiment appropriately, we can determine what information we can accurately gather during the analysis step.

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

sample preparation in quartz crystal microbalance measurements of protein adsorption and polymer mechanics

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.

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...

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

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.

In this study, we present various examples of how thin film preparation for quartz crystal microbalance experiments informs the appropriate modeling of ...

The quartz crystal microbalance is useful for sensing small concentrations of another compound and can be applied to questions regarding the mechanics of soft matter and biomaterial systems. The main advantage is that the quartz crystal microbalance is able to obtain highly accurate information for small sample sizes. When extracting mechanical property information, it is necessary to work with films of an appropriate thickness.Researchers must be careful to apply the correct modeling for a film during analysis. The technique is well-suited for studying a wide range of material systems and for understanding how mechanical properties of polymeric materials respond to the environment. After turning on all necessary equipment, remove the flow module from the chamber platform and unscrew the large thumbscrews to open the module.Dry the O-ring on the flow module with a stream of nitrogen gas and check that the O-ring is lying flat. Mount the sensor on the O-ring by placing the sensor with the active surface side down and an anchor shaped electrode oriented toward the marker in the flow module. Find the initial resonance frequencies of the sensor.Place the inlet pump tubing in the 1X PBS. Start the external pump flow at 25 microliters per minute and visually inspect the tubing to be sure that the fluid is flowing through the tube. Allow the fluid flow to properly equilibrate for at least 15 minutes.In the software, press start measurement to start the measurement and begin data acquisition. Monitor the frequency and dissipation values for at least five minutes to ensure a stable baseline. Stop the pump and move the inlet tubing to the collagen acetate buffer solution and resume fluid flow.Note the time of this event for later analysis. Allow the new frequency and dissipation values to equilibrate for eight to 12 hours to a stable value, then stop the pump, move the inlet tubing back to the 1X PBS solution and resume fluid flow. Note the time of this event for later analysis.Again, allow the new frequency and dissipation values to equilibrate for 30 minutes to a stable value. End the data acquisition of the measurement, then save the data. First, position a bare quartz crystal sensor in a sample holder connected to the vector network analyzer and computer.Turn on the analyzer to apply an oscillating voltage to the sensor and collect a reference conductance spectrum for the sensor in air, then submerge the sample holder in a lipless 100 milliliter beaker filled with distilled water and collect a reference conductance spectrum for the bare sensor in water. Insert a small silicon wafer into a 0.5 molar potassium bromide solution at an angle to create a slide for the quartz sensor during the annealing step to prevent the film from coming off of the sensor. Prepare to add the PEC to the surface of the sensor.If the PEC complex is in two phases, carefully draw up approximately 0.5 milliliters of the polymer-rich phase into the pipette. Maintaining pressure on the pipette bulb to not allow the polymer-poor phase to enter the pipette, draw the pipette out of the solution. Wipe the outside of the pipette using a chem wipe.Add enough solution drop wise onto the surface of the quartz sensor to completely cover the surface. Make sure there are no visible bubbles in the solution on the sensor surface. Spin coat the PEC sample and immediately submerge the sensor in the 0.5 molar potassium bromide solution to prevent salt crystallization on the film.Allow the film to anneal for at least 12 hours. Transfer the sensor to a beaker filled with distilled water to remove the excess potassium bromide from the film and the backside of the sensor. Leave the sensor in the solution for 30 to 60 minutes.Take a measurement of the film applied to the surface of the sensor in air referenced to the bare sensor in air. Allow the film data to equilibrate. Next, insert dried calcium sulfate into a 100 milliliter lipless beaker and measure the completely dry film thickness.Remove calcium sulfate from the beaker and rinse the beaker with distilled water. Fill the 100 milliliter lipless beaker with 30 milliliters of distilled water. Insert a stir bar to ensure the water is circulating around the film.With reference to the bare sensor in water, measure the film in water for about 30 to 45 minutes or until the film data are equilibrated. Prepare a 15 milliliter solution of three molar potassium bromide by measuring 5.35 grams of potassium bromide into a graduated cylinder and fill to 15 milliliters with distilled water. Swirl until dissolved.Face the film away from where the potassium bromide solution is being added to the water so that the film does not dissolve. Add the potassium bromide solution to the beaker with distilled water in 0.1 molar increments. Make sure the system is equilibrated before adding another addition of the potassium bromide solution.After all the data have been acquired, remove the film from the holder and place in a beaker of distilled water. Allow the salt to leave the film for 30 to 60 minutes and air dry the film. In this study, the introduction of collagen solution caused protein absorption to begin which was observed as a steady decrease in frequency and increase in dissipation over time until the density of adhered collagen plateaus at a stable baseline.The frequency changes are correlated with the mass of the film and the dissipation is correlated with energy dissipation from the film. A theoretical model is needed to obtain the viscoelastic properties quantitatively. The viscoelastic analysis of collagen using a power law model shows the aerial mass, complex shear modulus, and viscoelastic phase angle respectively.The first 10 hours indicate the main adsorption stage of the collagen to the sensor surface with the period between 10 and 20 hours showing the equilibration stage before the buffer wash performed at 20 hours. Here is the plot of the viscoelastic phase angle and the complex shear modulus over the general range of samples measured using the QCM. The green line indicates the linear relationship between the two properties.This plot shows how a PEC can demonstrate a wide range of mechanical properties based on the ratio of polymer, water and salt in the system. The sample preparation directly affects the results from a QCM experiment. Understanding what data you wish to learn from a sample informs how thick the sample should be.The QCM is very useful for sensing environmental and biological processes. It can also probe the rheological behavior in the high frequency regime for characterizing the viscoelasticity of materials.

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12.9K vistas.  Northwestern University. El microbalanza de cristal de cuarzo es útil para detectar pequeñas concentraciones de otro compuesto y se puede aplicar a preguntas relacionadas con la mecánica de la materia blanda y los sistemas de biomateriales. La principal ventaja es que el microbalance de cristal de cuarzo es capaz de obtener información altamente precisa para tamaños de muestra pequeños. Al extraer información de propiedades mecánicas, es necesario trabajar con películas de un espesor adecuado.Los inve...