The aim of this final section is to discuss and propose potential improvements for the previous alginate- and SF-based biosensors, following the same researching line.
Electrodeposited alginate based biosensors are considered highly developed in this thesis (Chapters 5.1 and 5.2), from the material characterization and biosensor optimization to the integration in a lab-on-chip type device using electrodes fabricated by Si-based technologies and PMMA structures mechanized by laser ablation. Furthermore, additional instrumentation design and production (µ-potentiostat) in collaboration with electronic engineers (ICAS group from IMB-CNM) have been developed for the biosensor control and data processing. The combination of the lab-on-a-chip and the instrumentation provides a fully-functional prototype only requiring minimal modification to be adapted to the requirement of the application of interest.
Figure 9.1. SF stamping process using PDMS moulds. SF patterning by stamping with PDMS. Previously, a layer of SU-8, deposited by spin coating (a) on a thermal oxidized Si wafer, is crosslinked by UV irradiation and developed with acetone (b).
Then, a PDMS precursor is casted over the SU-8 patterns (c), cured at 80 ºC and peeled off (d). A SF solution drop is casted on a glass substrate (e) and the PDMS pressed over it until the water evaporates (f). The SF patterns are finally water annealed (g).
In case of SF, also a deep study has been carried out from a macroscale point of view. It has demonstrated to be biocompatible for the immobilization of enzymes with electrochromic or photoelectrochromic compounds for the development of a glucose
optical biosensor. In fact, the first reversible optical biosensor has been developed in the framework of the present thesis based on the use of photoelectrochromic biosensors.
Nevertheless, the material has been used only as plane doped films integrated in PMMA structures or as a doped cladding for PMMA filaments, either by direct casting of the precursor solution or by dip coating of the filaments (Chapters 6.3 and 6.6).
Following these results and as a future approach, first steps have been taken for the micropatterning of SF with the aim of integrating the material in clean room processes.
In a first indirect approach, the SF has been microstructured by stamping on a glass substrate, using a PDMS negatively patterned mould. The PDMS mould is previously obtained by casting on a SU-8 structure fabricated in a photolithography process (Figure 9.1). Good resolutions are achieved, as it is shown in the microscope image (Figure 9.2a) and in the 3D reconstruction (Figure 9.2b).
Figure 9.2. SF waveguides fabrication. a) Optical microscope image from the top and b) 3D reconstruction from AFM results of a SF waveguide fabricated by stamping (axis in µm). c) Light confinement in a waveguide and d) planar waveguides scheme on a substrate.157
This protocol can be potentially used in planar waveguides fabrication due to the relatively high RI of SF (~1.54), what would permit an effective light coupling in the
structure (Figure 9.2c, 9.2d).157 Furthermore, the biocompatibility of the SF patterning process drives to the possibility of doping the bulk of the waveguides with biorecognition elements and colorimetric molecules, in contrast to the extensively used planar waveguides of SiO2 or Si3N4. As it has been demonstrated in this thesis, doped SF films and claddings permit the diffusion of small analytes (i.e. glucose), changing the color due to enzymatic reactions occurring within the matrix, while retaining potential interfering molecules, such as haemoglobin o the cell fraction. The same strategy can be applied to planar waveguides, where the increased optical path length compared to the thin films used in this thesis, would increase the sensibility of the biosensor.
Figure 9.3. SF patterning by e-beam. a) SEM image of e-beam patterned SF layers of 200 nm thickness deposited by spin coating on a thermally oxidized Si wafer. The numbers below the squared irradiated areas indicate the factor (from 0.1 to 100) applied from a basic dose of 100 µC cm-2 (0.1 factor corresponds to 10 µC cm-2). b) Detail of an 800 µC cm-2 irradiated area and c) respective profile measured by AFM.
Additionally to stamping patterning, also e-beam lithography has been explored as an alternative strategy to structure SF films. This method has demonstrated to pattern SF both as a positive or as a negative resist.162 If the SF is previously annealed, it can be used as a positive resist with high irradiation doses (Figure 9.3), while if it is not annealed, the crosslinking can be provoked by low doses of e-beam irradiation. This method is also all-water based and, therefore, compatible with biomolecules immobilization, especially when the SF is used as positive resist as no irradiation occurs over the remaining material.
The use of the presented techniques opens the possibility of SF structuring not only for light guiding and optical biosensing applications but also, for example, for highly controlled fluidic management based on the microfluidic synthetic paper concept developed by Prof. van der Wijngaart at KTH using OSTE. In this concept, the construction of micropillars enhances de capillary pumping properties of the material that, thanks to the precision of microfabrication techniques, can be highly controlled by playing with the size and distribution of the pillars.
The excellent optical properties of SF for its application in optical biosensing, demonstrated in this thesis, can be therefore improved by it microstructuration, reducing the amount of material and dopants used, miniaturizing the systems, and increasing the capillary properties to achieve faster detections.