Single Molecule Studies of Solvent-Dependent Diffusion and Entrapment in Poly(dimethylsiloxane) Thin Films

Abstract

Single molecule microscopic and spectroscopic methods are employed to probe the mobility and physical entrapment of dye molecules in dry and solvent-loaded poly(dimethylsiloxane) (PDMS) films. PDMS films of approximately 220 nm thickness are prepared by spin casting dilute solutions of Sylgard 184 onto glass coverslips, followed by low temperature curing. A perylene diimide dye (BPPDI) is used to probe diffusion and molecule-matrix interactions. Two classes of dye-loaded samples are investigated: (i) those incorporating dye dispersed throughout the films ("in film" samples) and (ii) those in which the dye is restricted primarily to the PDMS surface ("on film" samples). Experiments are performed under dry nitrogen and at various levels of isopropyl alcohol (IPA) loading from the vapor phase. A PDMS-coated quartz-crystal microbalance is employed to monitor solvent loading and drying of the PDMS and to ensure equilibrium conditions are achieved. Single molecules are shown to be predominantly immobile under dry conditions and mostly mobile under IPA-saturated conditions. Quantitative methods for counting the fluorescent spots produced by immobile single molecules in optical images of the samples demonstrate that the population of mobile molecules increases nonlinearly with IPA loading. Even under IPA saturated conditions, the population of fixed molecules is found to be greater than zero and is greatest for "in film" samples. Fluorescence correlation spectroscopy is used to measure the apparent diffusion coefficient for the mobile molecules, yielding a mean value of D = 1.4(+/-0.4) x 10(-8) cm(2)/s that is virtually independent of IPA loading and sample class. It is concluded that a nonzero population of dye molecules is physically entrapped within the PDMS matrix under all conditions. The increase in the population of mobile molecules under high IPA conditions is attributed to the filling of film micropores with solvent, rather than by incorporation of molecularly dispersed solvent into the PDMS.

Single-Molecule Studies of Oligomer Extraction and Uptake of Dyes in Poly(dimethylsiloxane) Films

“…26 These same simulations were also used to verify the dynamic range and integrity of the analysis under conditions that mimicked those of the experiments. 17 In the second method of analysis, the total number of fluorescent "bursts" above background was counted along the fastscan direction in each image. Bayesian statistical methods were employed to ensure proper counting of the bursts.…”

“…6,16 Recently, we reported the first investigation of cured PDMS materials by single-molecule (SM) spectroscopic methods. 17 In this previous investigation, we studied molecular mobility and entrapment within PDMS films exposed to isopropyl alcohol (IPA) vapors. We showed that, as the IPA content of the ambient environment increased, the population of mobile dye molecules increased, while a reduced (but nonzero) fraction of physically entrapped molecules remained.…”

“…Samples were extracted 5, 10, 20, and 40 times, using a previously developed spin extraction process. 17 The films were loaded with nanomolar concentrations of perylene diimide (N,N′-bis(butoxypropyl)perylene-3,4,9,10-tetracarboxylic diimide, BPPDI; a neutral, hydrophobic dye) and cationic rhodamine 6G (R6G; a cationic, hydrophilic dye) for SM studies. Images of the different samples incorporate fixed, round fluorescent spots that are attributed to entrapped SMs.…”

Single-Molecule Perspective on Mass Transport in Condensed Water Layers over Gradient Self-Assembled Monolayers

“…The resulting gradient SAM films looked optically clear and clean by eye. Each film was then stored in a desiccator under static vacuum for 12 h. These were subsequently spin coated (5000 rpm, 30 s) with a 150 μL aliquot of 4 nM C 11 OPDI dye (synthesized in house) , in HPLC-grade ethanol. After coating with dye, each gradient was stored in a desiccator for an additional 48 h before characterization by SMT and other methods.…”

Detection and identification of single molecules

“…An important role is played by the modern methods of nanotechnology, including scanning tunnel microscopy, which under certain conditions acts as scanning tunnel spectroscopy. Many similar facts are considered in the big review [1]; other examples can be found in recent publications [4][5][6][7]. Thus, nanosize devices based on one-wall carbon nanotubes ( CNT ) are sensitive to single molecules, in particular hydrogen peroxide and melphalan (alkylating agent) [4].…”