Su 8 2000

The microfluidic chips were designed
using AutoCAD (Autodesk) and printed out on a high-resolution film
photomask (Micro Lithography Services). The mask designs are published
on dropbase (https://openwetware.org/wiki/Dropbase:_Double-emulsion-02). The master molds of microfluidic devices were fabricated following
standard hard lithography protocols. First, 15-μm-high microfluidic
structures were patterned on 3 in. silicon wafers (Microchemicals)
using high-resolution film masks and SU-8 2015 photoresist (Kayaku
Advanced Materials) according to the guidelines of the manufacturer
(SU-8 2000, Micro Chem). A MJB4 mask aligner (SÜSS MicroTec)
was used to UV expose all of the SU-8 spin-coated wafers. The thickness
of the structures (corresponding to the depth of channels in the final
microfluidic devices) was confirmed by measurement with a Dektat stylus
profilometer (Bruker).

The PI encapsulated devices (HBT, Schottky diode, inductor and capacitor, and digital electronics) on temporary Si substrates were boiled in acetone at 200 °C for 2 h to remove the underlying sacrificial layer (PMMA). A large PDMS elastomer stamp was used to transfer print the finished devices onto the CNF substrate with a thin layer of polymer (SU8 2000.5, Microchem, 500 nm) as the adhesive layer to ensure good bonding between the CNF substrate and the transferred devices.

For this experimental work we used as photonic transducer a Biophotonic Sensing Cell (BICELL) [8 (link)–10 (link)] based on Fabry-Perot interferometers of a square size of 1 cm² and on SU8 polymeric thin film that exhibits a reliable optical label-free biosensing capability. For the BICELL fabrication we use SU8 2000.5 (MicroChem Corp., Newton, MA, USA) diluted 1:10 in cyclopentanone; this SU8 was deposited by spinning and then soft-baked at 70 °C for 1 min. An exposure to UV light process was then carried out, followed by a post-bake step at 70 °C for 5 min to give a stable thin film. Finally, an acid catalysis step was carried out in order to increase the hydrophilicity of the film. The label-free optical biosensing is carried out by monitoring the changes in the optical mode response provoked by the occurence of both the immobilization of the anti-dengue bioreceptors and the recognition of dengue virus antigen.

Gold(III) chloride trihydrate (≥99.9% trace metals basis)/Au salt with the chemical formula HAuCl ${}_4$ .3H ${}_2$ O was purchased from Sigma-Aldrich. SU-8 2000.5 (epoxy-based negative photoresist) was purchased from MicroChem Corporation.
Au NPs were fabricated inside a polymer film deposited on a glass substrate following the process shown in Figure 1. The general procedure follows three steps, all of them in air environment conditions: (i) Au salt was mixed with SU-8 2000.5 at different weight ratios (wt.%) by stirring for 20 minutes for complete dissolution. (ii) The nanocomposite metal/resist solution was then deposited on a glass substrate by spin-coating at 500 rpm for 5 s and then at 2000 rpm for 30 s. (iii) After that, thermal annealing treatment was carried out at different temperatures, between room temperature and 240 ${}^{°}$ C (accuracy ±1 ${}^{°}$ C), using a standard hot plate.
The nanocomposite film thickness was measured using a profilometer in a clean room, and ranged from 500 to 1000 nm. The plasmonic color can be observed by eye and by using a standard camera combined with an optical microscope. The plasmonic properties of the Au NPs in SU-8 resist were characterized by an ultraviolet–visible (UV–Vis) spectrometer. To evaluate Au NPs sizes, shapes, and distributions, the nanocomposite was dropped on a carbon-coated Cu grid and examined by a transmission electron microscope (TEM).

First, a negative resist SU-8 2000.5 (Microchem Corp.) was spun at 3000 rpm on a 100 nm thick Al film, which was previously deposited on a Si substrate, and soft-baked at 110 °C for 1 min on a hot plate. Next, a 600 nm period square lattice of circular solid nanodots was written in the resist film by electron beam lithography (EBL) at 50 kV and 50 pA in a Crestec CABL-9000C high-resolution EBL system [9 ]. After electron beam exposure, the samples were post-baked at 80 °C for 3 min to crosslink the SU-8 irradiated regions. Next, the part of the resist that was not crosslinked was removed by rinsing the samples in MicroChem SU-8 developer at −15 °C for 7 s and then gently dried with nitrogen flow. This resulted in ≈270 nm diameter and ≈320 nm tall SU-8 nanopillars, as shown in the scanning electron microscope (SEM) photograph of Figure 1a. Then, a thin layer of Al was electron-beam evaporated (deposition rate = 1 nm/s) on the SU-8 nanopillar array (Figure 1b). The measured thickness of the deposited Al film on a flat substrate was 40 nm. Finally, the sample was exposed to an oxygen plasma (RF power = 50 W, flow rate = 15 sccm) for 30 min (Figure 1c).