Functionalized Electrospun Nanofibers In Microfluidic Bioanalytical Systems

Abstract

Biosensors detect target analytes through specific binding with biological recognition elements such as nucleic acids, enzymes, and antibodies. Many labs are working to create inexpensive and portable miniaturized sensors that allow for rapid sample analysis and low reagent consumption in order to increase biosensor accessibility in rural areas and third world countries. Lab-on-a-chip devices aim to incorporate sample preparation and analyte detection into one device in order to create self-contained sensors that can be used in rural areas and third world countries where laboratory equipment may not be available. Often, these devices incorporate microfluidics in order to shorten reaction times, reduce handling of hazardous samples, and take advantage of laminar flow [1]. However, while several successful lab-on-achip devices have been developed, incorporating sample preparation and analyte detection within one device is still a key challenge in the design of many biosensors. Sample preparation is extremely important for miniaturized sensors, which have a low tolerance for sample impurities and particulates [1]. In addition, significant sample concentration is often required to reduce sample volumes to the nL to mL range used in miniaturized sensors. This research aims to address the need for sample preparation within lab-on-a-chip systems through the use of functionalized electrospun nanofibers within polymer microfluidic devices. Electrospinning is a fiber formation process that uses electrical forces to form fibers with diameters on the order of 100 nm from polymer spinning dopes [2, 3]. The non-woven fiber mats formed during electrospinning have extremely high surface area to volume ratios, and can be used to increase the sensitivity and binding capacity of biosensors without increasing their size. Additionally, the fibers can be functionalized through the incorporation of nano and microscale materials within a polymer spinning dope. In this work, positively and negatively charged v nanofibers were created through the incorporation of hexadimethrine bromide (polybrene) and poly(maleic anhydride) (Poly(MA)) within a poly(vinyl alcohol) spinning dope. Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed the successful incorporation of polybrene and poly(MA) into the nanofibers. Gold microelectrodes were patterned on poly(methyl methacrylate) (PMMA) to facilitate the incorporation of nanofibers within microfluidic devices. The gold microelectrodes served as grounded collector plates during electrospinning and produced well-aligned nanofiber mats. Microchannels 1 mm wide and 52 [MICRO SIGN]m deep were imprinted into PMMA through hot embossing with a copper template. PMMA pieces embossed with microchannels were bonded to PMMA pieces with gold microelectrodes and nanofibers using UV-assisted thermal bonding. Positively charged polybrene-modified nanofibers were shown to successfully filter negatively charged fluorescent liposomes out of a HEPES-sucrose-saline buffer, while negatively charged poly(MA)-modified nanofibers were shown to repel the liposomes. The effect of nanofiber mat thickness on liposome retention was studied using the z-scan function of a Leica confocal microscope. It was determined that positively charged nanofibers exhibited optimal liposome retention at thicknesses of 20 [MICRO SIGN]m and above. Negatively charged nanofiber mats over 40 [MICRO SIGN]m thick retained liposomes due to their small pore size despite their surface charge. Finally, it was demonstrated that a HEPES-sucrose-saline solution of pH 8.5 could be used to change the charge of the positively charged polybrene nanofibers and allow for the release of previously bound liposomes. The results of this study can be used to design lab-on-a-chip devices capable of performing all sample preparation and analyte detection in one miniaturized microfluidic sensor. vi In addition, other nanofiber surface chemistries can be studied to create more specific sample filtration and allow for immobilization of biological recognition element. vii