Nanoprobe Diffusion in Poly(Vinyl-alcohol) Gels and Solutions: Effects of pH and Dehydration
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Nanoprobe Diffusion in Poly(Vinyl-alcohol) Gels and Solutions: Effects of pH and Dehydration 1
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Hacène Boukari, 2Candida Silva, 2Ralph Nossal, and 2Ferenc Horkay
1 Department of Physics and Engineering, Delaware State University, Dover, DE 19901 Eunice Kennedy Shriver National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892
ABSTRACT We report fluorescence correlation spectroscopy (FCS) measurements of the translational diffusion of two fluorescent nanoprobes, rhodamine (R6G) and carboxytetramethylrhodamine (TAMRA), embedded in poly(vinyl alcohol) (PVA) solutions and gels. The diffusion coefficient was measured as a function of the PVA concentration and pH. Furthermore, we designed and built an optical chamber to determine the diffusion coefficient of the nanoprobes within the PVA solutions and gels subjected to controlled dehydration. We find that 1) lowering pH causes an apparent slowing down of the diffusion of the nanoprobes, 2) increase of PVA concentration and crosslink density also induce slowing down of both nanoprobes, and 3) dehydration induces systematic decrease of the diffusion of TAMRA in both solutions and gels. Taken together, these results demonstrate that transient physical interactions between the nanoprobes and the PVA linear polymers have a significant effect upon nanoprobe diffusion.
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I. INTRODUCTION Hydrogels are essential for biomedical applications. Examples of their use are engineering of tissue phantoms, designing extracellular matrices for tissue regeneration, and developing efficient drug delivery systems [1-5]. Understanding their structural and dynamical properties is therefore necessary, especially with the growing demand for hydrogels tailored for specific applications. In particular, the sieving properties of gels remain poorly understood despite numerous investigations at both theoretical and experimental levels. A general consensus on the parameters that control the transmissibility of various materials through the gels has yet to emerge. Recently, we demonstrated how fluorescence correlation spectroscopy (FCS) could be applied to study the diffusion of various nanoprobes – both small fluorophores and nanoscopic fluorescent biomacromolecules – in polymer solutions and gels [6,7]. We reported FCS measurements of various fluorescent nanoprobes (e.g. BSA, Phycoerythrin, dextran, polystyrene beads) in differing PVA solutions, and showed that the measurements can be analyzed using De Gennes’ model [8]. Two relevant length scales, the size of the nanoprobe (d) and the mesh size ([) of the host polymer system, were identified. In PVA solutions, when the size of the nanoprobe is comparable to the mesh size of the solution, the dependence of the apparent diffusion coefficient of the nanoprobes on the PVA concentration can be described by a stretched exponential, the exponent being related to the solvent quality [7,9,10]. In PVA gels, we found a strong correlation between the elastic modulus of the gel and the apparent diffusion t
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