The science behind the buildup mechanism of polyelectrolyte multilayers is important for developing devices for various engineering applications. Here we, study the dependency of thickness of polyelectrolyte multilayer films, fabricated using spin-assisted layer-by-layer self-assembly of polyelectrolytes technique, with respect to varying spin-speed while keeping all other parameters of the fabrication process-window constant. The thickness measurements were performed using variable angle spectroscopic ellipsometry and atomic force microscopy. The experimentally observed results were validated mathematically using a Flory type theory. In addition, the biomolecular adsorption studies on these polyelectrolyte multilayer films fabricated at various spin-speeds, were also quantitatively analyzed using fluorescence microscopy studies. It was seen that the effect of spin-speed on the thickness of polyelectrolyte multilayers was negligible. In addition, it was also observed that the bio-molecular adsorption modalities onto these substrates were also independent of the spin-speed. This finding prompts to develop low-cost alternative technologies for various biomedical engineering applications, like functionalized substrates for centrifugal assay for fluorescence-based cell adhesion, wherein stability of films against strong mechanical forces generated during spinning can play an important role.

Layer-by-layer (LbL) self-assembled polyelectrolyte multilayer (PEM) films are a simple yet elegant bottom-up technology to create films at the nano−microscale. This low-cost technology has been widely used as a universal functionalization technique on a broad spectrum of substrates. Biomolecules under investigation can be incubated onto films based on complementary charge interactions between the films and biomolecules. There is a great demand for developing an ultralow-cost biosensing device, which can optimally enhance the fluorescence signal of the adsorbed biomolecules from the traditional labeled sensing platforms. In this work, we have incorporated a blend of the conventional metal enhanced fluorescence technology and the PEM as a dielectric spacer and functionalized film, coated on an aluminum paper (tape)-based substrate. These device has been found to be capable of holding biomolecules in three-dimensional PEM space. The devices fabricated by the proposed spray LbL technique provide significant fluorescence signal enhancement by holding a relatively higher mass per volume of the adsorbed biomolecules, when compared to traditional spin-and dip-coating techniques. Interestingly, our proposed device has expressed a fluorescence enhancement factor, which is 9 times higher than PEM-functionalized glass-based devices. To demonstrate the practical utility of our devices, we also compared our devices to Whatman FAST slides. Our experimental fluorescence results are almost comparable to Whatman FAST slides. Such PEM devices fabricated on top of low-cost aluminum tape using a spray LbL technique give new insights into the future development of ultralow-cost, high-throughput, and disposable lab-on-chip diagnostic applications.

Polyelectrolyte multilayer (PEM) films fabricated using Layer-by-Layer (LbL) self-assembly are most versatile for numerous surface tailoring applications. Physicochemical customization of PEM at a nanometer scale is possible by choice of incorporated materials, assembly parameters, and assembly techniques. Conventional dip (CDip) (10-15 min dip time) coating dominates the PEM fabrication due to its simple, geometry-independent, and economical coating capabilities. Deposition of the PEM in CDip-LbL happens in two phases, that is, the initial adsorption phase followed by rearrangement of the complementary polyelectrolyte chains. By shortening the prolonged rearrangement phase, thicker stratified PEM films can form with a reduced dipping time (1 min), called Fast Dip (FDip). This reduction in the dipping time is studied in situ using Etched Fiber Bragg Grating-based sensors. To define the practical utility of the FDip deposited PEM, bio-molecular loading studies are performed using biotin and Bovine Serum Albumin (BSA). FDip-LbL is as effective as CDip-LbL self-assembly and also better than spin-LbL. Thus, the FDip deposition prompts a scalable LbL self-assembly process for quick fabrication of the biosensing platforms.