Tuning the Effective Viscosity of Polymer Films by Chemical Modifications

Self-assembly strategy to reduce non-specific adsorption for the development of high sensitivity quantitative immunoassay

The development of functionalized surfaces with low non-specific adsorption is important for their biomedical applications. To inhibit non-specific adsorption on glass substrate, we designed a novel optical biochip by modifying a layer of dense negatively charged film (SO₃^2-) on its substrate surface via self-assembly. Compared with the untreated glass substrate, it reduced the adsorption by about 300-fold or 400-fold by poly (styrene sulfonic acid) sodium salt (PSS), or meso-tetra (4-sulfonatophenyl) porphine dihydrochloride (TSPP) on individually the modified glass substrate. Considering the effect of fluorescence resonance energy transfer (FRET) between TSPP and the QDs in solution by mixing, a strategy of 2-layer of TSPP followed by 4-layer of PSS was designed to modify the glass for preparing biochips. Under the optimized conditions, the biochip on functionalized glass substrate co-treated with TSPP and PSS realized the sensitive quantitative detection of C-reactive protein (CRP) based on a quantum dot fluorescence immunosorbent assay (QD-FLISA). The limit of detection (LOD) for CRP achieved 0.69 ng/mL with the range of 1-1,000 ng/mL using TSPP and PSS co-treated glass substrate surface, which was respectively about 1.9-fold and 7.5-fold more sensitive to the PSS-modified biochip and the TSPP-modified biochip. This work demonstrated an effective and convenient strategy to obtain biochips with low non-specific adsorption properties on functionalized surfaces, thus providing a new approach for creating ultra-high sensitivity microchannels or microarrays on glass substrates.

Strain Rate and Thickness Dependences of Elastic Modulus of Free-Standing Polymer Nanometer Films

Elastic moduli, E, of free-standing polystyrene (PS) single-layers and polystyrene-polydimethylsiloxane (PS-PDMS) bilayers are measured by uniaxial tensile testing at room temperature under different strain rates, γ̇, and for PS thicknesses, h, from 8 to 130 nm. As γ̇ increases, E increases initially, then approaches the bulk value, E_bulk, when γ̇ exceeds a characteristic value (≡ τ^-1) that decreases with increasing h. The noted variation of E with γ̇ shows that stress relaxation occurs in the films during measurement when γ̇τ ≪ 1, while the noted variation of τ^-1 with h shows that thinner films relax faster. Consequently, E decreases with decreasing h if γ̇ is small, but displays independence of h if γ̇ is large. Visually, the crossover takes place at around γ̇ = 0.0015 s^-1, where at γ̇τ > 1 for all films.

Tg Confinement Effect of Random Copolymers of 4-tert-Butylstyrene and 4-Acetoxystyrene with Different Compositions

We observe that the T_g confinement effect of polymer films can saturate with polymer-substrate interaction. Thickness dependences of the glass transition temperature, T_g(h₀), of random copolymer films of 4-tert-butylstyrene (TBS) and 4-acetoxystyrene (AS) supported by silica (SiOx) were measured for different TBS concentrations, X_TBS. For 0 ≤ X_TBS ≤ 0.47, T_g(h₀) displays identical enhancements, independent of X_TBS. For X_TBS > ∼0.66; however, T_g(h₀) decreases steadily with X_TBS. The X_TBS > 0.66 result is in keeping with expectations that TBS interacts less strongly with SiOx than AS does, and weaker polymer-substrate interaction renders greater dominance of the air surface over substrate surface on T_g, and thereby T_g reduction. We propose that saturation in T_g(h₀) found for X_TBS ≤ 0.47 is caused by the maximization in polymer-substrate-specific bond formation. Further experiments and a calculation support this proposition.