Templating gold nanoparticles on nanofibers using block copolymer thin films

Surface enhanced Raman scattering (SERS) nanosensors based on pulled nanofibers are a promising technology for cellular studies. A bottom-up nanofabrication approach towards SERS nanofibers is based on electrostatic self-assembly of charged gold nanoparticles (AuNPs) with a film carrying opposite charges that coats the nanofiber. Common coatings such as self-assembled monolayers of alkoxysilanes and homopolymer films suffer from AuNP aggregation and inhomogeneities. Block copolymer (BCP) film templates having surface patterns can guide the ordered deposition of AuNP arrays, but have been investigated almost exclusively on flat substrates. A major objective of this thesis is to coat nanofibers with BCPs with the aim of achieving uniform AuNP arrays on the nanofiber.  To achieve this, pulled glass fibers with sub-micron tip diameters were dip-coated from polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) solutions in THF of various polymer solution concentrations and dip-coating rates. It was found that only a brush film with a thickness of about 3 nm was obtained on the nanofiber. The brush film forms by adsorption, where the P4VP block anchors the BCP on the polar surface of the glass nanofiber via multiple H-bonds and the PS block dangles into the solution. Incubating the BCP-coated nanofiber in aqueous AuNP colloids leads to the deposition of dense and well-dispersed AuNPs on the nanofibers. In contrast, P4VP homopolymer and PS-s-P4VP random copolymer films show significant AuNP aggregation, indicating a steric barrier effect provided by the PS blocks. An increase in roughness of the BCP brush film was shown after immersion in water, indicating that the AuNP deposition results from local PS chain rearrangement to expose portions of the P4VP chains for attracting AuNPs. Similarly, when following the adsorption kinetics by measuring the dry film thickness as a function of adsorption time, it was found that saturated films had a smooth morphology (achieved within two minutes of adsorption) and unsaturated films exhibited a rougher morphology, but this did not change the deposition pattern of the AuNPs.  For greater understanding, the same dip-coated PS-b-P4VP films were studied on flat surfaces. The effect of surface curvature (fiber diameter) was found to parallel that of changing polymer solution concentration, as indicated by a similar film morphology evolution between decreasing polymer solution concentration and decreasing fiber diameter. Notably, it changes from a full micellar morphology to a mixed morphology of micelles and the brush layer (dewetting) to a full brush layer. While the featureless brush films led to dense and well dispersed AuNP deposition, the dots led to small AuNP clusters for AuNP diameters of about 10 nm. Finally, it was shown that nanofibers can be coated with gold ion-loaded BCPs using drop-coating techniques. In terms of applications in SERS optophysiology, the BCP brush-coated nanofiber with well dispersed AuNP deposition was shown to give a superior performance, as shown by in vitro detection of neurotransmitters.

Keyphrases: SERS, Adsorption, Self-Assembly, Gold Nanoparticles, Substrate Curvature, Dip-Coating, Block Copolymer Adsorption, Block Copolymer Brush Layers, Polystyrene-block-poly(4-vinylpyridine), Block Copolymers, Couches Minces, Thin Films