When the quartz crystal microbalance (QCM) is operated in contact with solution and used to detect inertia increases caused by macromolecules binding to its surface, resonance frequency shifts are reported in the literature to be greater than, less than, and the same as an identical macromolecular mass would cause as a dry layer. A previous report of wet and dry M13 DNA giving the same, linear frequency versus mass response is examined. The M13 data are shown to follow the reciprocal of the square root of mass, not the reported linear relationship. New experiments on RNA duplexes oscillated in solution are reported. A lossy polymer layer is placed between the QCM and RNA. When changes in density, viscosity, and included water are eliminated, the response remains linear for a constant adlayer thickness. The expectation that response per unit mass should decrease with distance from the QCM surface is demonstrated. Total decoupling of mass lying beyond the acoustic overlayer is also demonstrated. The present results are placed in context with recently published results from a study of progressively thicker protein layers bound to the QCM.
Aqueous solutions of alkylamines and 95% ethyl alcohol will react with the acid chloride of polyethylene-co-acrylic acid to form amides and ester, respectively. Chemical and spectroscopic evidence, as well as contact angle measurements, shows that the reactions are due to formation of acid chloride groups below the polymer's surface. A film of SOCl2-treated polyethylene-co-acrylic acid retains its reactivity to amines and alcohol even after long soaking in aqueous base. Alkylamines and ethyl alcohol readily penetrate into the polymer film where they react with interior acid chloride groups, while water and charged or large molecules do not. To explain the film's properties, a three-layer model of its morphology is proposed. Practical application of the acid chloride film to the tethering of aminoalkyl-5‘-modified oligo-DNA is demonstrated. A site-specific reaction tethers the oligo-DNA to the film exclusively via its aminoalkyl tail. The reaction occurs spontaneously when the acid chloride film is allowed to soak in an aqueous solution of oligo-DNA at pH 11.5. Spectral and contact angle evidence and the reactions of model compounds indicate that the terminal amino group on the modified oligo-DNA reacts with an internal acid chloride group to form an amide bond. A highly active, surface-tethered oligo-DNA results.
Yoshimoto et al. [Anal. Chem. 2002, 74, 4306-4309] reported that a quartz crystal microbalance or QCM changed its response to sucrose solutions according to its angle of immersion. The effect was tentatively attributed to gravity-caused stress on the viscous interface between the oscillator and the bulk solution. The present work reports results from QCM experiments carried out so that any effect of gravity on the interfacial region would be magnified. This permitted use of a lower-frequency, less-sensitive QCM. Molecules of DNA were tethered to a functionalized QCM surface and then extended in steps, via sandwich hybridization, to produce DNA of uniform and known length. This feature allowed both the effect of QCM immersion angle and the relationship between frequency and molecular length to be investigated simultaneously. Comparison of acoustic wave damping at 0 degrees and 180 degrees immersion angles offers compelling evidence that the interfacial region expands when the active face of the QCM is down and contracts when it is up. This is apparently a consequence of the interfacial region being more dense than the bulk solution. The results are consistent with (a) slow gravity-driven movement of molecules away from a down-facing QCM, (b) rapid hybridization-driven movement away from an up-facing QCM, and (c) a QCM frequency response that decreases according to a simple exponential function of the tethered molecules' radius of gyration.
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