Optofluidic liquid sensing on electromicrofluidic devices

Figure 1(a) shows that the EMF device consists of two parallel plates with an unpatterned ground electrode on the top plate, the patterned driving electrode on the bottom plate and a spacer of 50 μm between them. We used a top glass plate (thickness 0.7 mm) coated with transparent indium tin oxide [39] (ITO, thickness 200 nm) to enable optical analyses and direct observation during the experiments. To facilitate liquid handling, the top ITO glass was spin-coated with a hydrophobic layer (Teflon^® AF 1600, DuPont, thickness 120 nm). The bottom black BT (Bismaleimide Triazine) resin plate with Cu driving electrodes patterns was covered with a dielectric layer. It was manufactured by Chip Win Technology Co., Ltd, Taiwan. A Teflon layer (thickness 120 nm) was also coated on the bottom plate to make the surface hydrophobic. Before assembly of the two plates, the liquids were manually pipetted on the bottom plate; the top plate was then carefully placed and adhered onto appropriate spacers (thickness 50 μm) attached to the bottom plate. The multiple driving electrodes on the bottom plate were individually activated through single pole double throw (SPDT) relays (LU-5, Rayex Electronics). The electric potential of the electrodes was switched between the electric ground and high potentials. AC electric signals with a frequency of 1 kHz were generated by a function generator (33210 A, Agilent Technologies) and amplified through an amplifier (A-304, A. A. Lab Systems). The relays were switched by the digital output signals of a data-acquisition device (USB-6509, National Instruments) programmed with LabVIEW software.

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To incorporate the porous BM onto the EMF chip we proceeded to its deposition by PV-OAD on the desired driving electrode (from now on the BM-electrode) of the chip by covering the rest with an aluminum shadow mask. Simultaneously, deposition by electron beam evaporation was also performed on ITO substrates and on silicon wafers. As reported in previous works alternating layers were stack deposited by evaporation at a zenithal angle of deposition (α) of 70° [36, 37]. The BM stack consisted of 15 alternating layers of TiO₂ and SiO₂, starting and ending with TiO₂, each of them with a thickness of about 85 nm except for the central layer that, acting as optical defect, had a thickness of ca. 200 nm.

Reflectance-visible spectra of the different parts of the device including the BM-electrode were taken using a home-made optical setup consisting of an extensive source of light (range 350–900 nm, Halogen Reflector Lamp Decostar Osram^®) collimated to a spot of around 1 mm² and a camera connected to a computer with a detector placed in the center of the camera. The impinging and reflected beam directions were 20° apart (incidence 10° off normal). Spectra were recorded before and after infiltrating the BM with liquids either dripped manually with a pipette or moved to the BM-electrode by EWOD or DEP actuation. The room temperature during the experiments was 20 °C.

A silicon wafer covered with the BM was diced to take Scanning Electron Microscopy (SEM) cross section micrographs in a Hitachi S4800 field emission microscope working at 2 keV primary beam energy. Both detecting modes, Secondary Electron (SE) and Back Scattered Electron (BSE) were used to highlight either topographic or compositional information, respectively.

Figure 1(b) shows SEM micrographs of the BM deposited onto a silicon wafer, together with an overprinted scheme of the layers stack. The BSE micrograph reveals the composition of each layer of the stack with the TiO₂ corresponding to the brighter layers and the SiO₂ to the darker ones. Meanwhile, the SEM micrograph shows the typical slanted nanocolumnar microstructure of the PVD-OAD films and highlights the high porosity of these layers that in previous works we have estimated around 50%. Figure 1(c) presents the EMF device with the BM deposited on a pre-selected driving electrode (the BM-electrode) that remained uncovered by the mask during deposition. This part presented an orange color that was taken as hint of the successful deposition of the BM, providing a straightforward way to localize it onto the device. An optical characterization of the different parts of the EMF device in the form of reflectance spectra is shown in figure 1(d), together with an equivalent spectrum of a reference BM sample deposited onto an ITO substrate which is included for comparison. This reference spectrum depicts the typical wide reflectance gap and resonance peak of this type of 1D photonic structure (blue line in Figure 1(d)). As reported in previous works [36–38], this resonant peak at around 460 nm can be used to monitor changes in the BM when it is liquid-infiltrated. The reflectance spectrum recorded for the BM deposited onto the EMF chip depicts a similar spectrum where the resonant peak was less well defined and appeared slightly shifted to shorter wavelengths (red line in Figure 1(d)). We attributed this slight loss of optical quality for the BM deposited onto the EMF device to the roughness of the driving electrode. Optical spectra of the black background and of a driving electrode of the EMF, also reported in Figure 1, were recorded to identify their possible contributions to the BM spectrum in case of misalignment of the incident beam. The high reflectivity of the BM-electrode zone in comparison with the black background and a driving electrode of the DMF device could be appreciated with the bare eyes in the dashed blue square of Figure 1(c) and was characterized by the spectrum presented in Figure 1(d).

Figure 2 presents the optofluidic characterization of the BM-electrode when delivering the liquids directly onto the BM with a pipette. Dripping liquids onto this zone should fill the pores of the BM and, according to previous analysis, would induce changes in the optical response of the BM transducer [36–38], Effectively, Figure 2(a) shows that the infiltration with different liquids led to a shift of the optical spectra to larger wavelengths (i.e., a redshift), that in the case of mineral oil reached up to 21 nm. The representation of the magnitude of these shifts as a function of the refractive index of the infiltrating liquid in Figure 2(b) shows a clear dependence that can be used to differentiate a specific liquid from others with different refractive index. The plot shows that below a RI of 1.38 the accuracy of the sensor decreases preventing to discriminate liquids below this point.

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Isopropanol (mainly driven by EWOD) and silicone oil (mainly driven by DEP) were used to determine the working parameters of the EMF device to move liquid droplets from one driving electrode to the other on the chip. Droplets of these liquids were moved by electrical actuation and the displacement from one electrode to another took around 0.2 s. A square wave signal of voltage 360 V_pp and a frequency of 1 kHz were supplied to move the liquid droplets, as well as to split the liquid into small droplets of volume $\unicode{x00303}$ 0.2 μl. A succession of pictures showing the formation of specific silicone oil patches in the EMF-device is presented in Figure 3.

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Main objective of this work was to investigate the ability of liquid droplet manipulation on an EMF device incorporated with a liquid transducer. To that end, a big silicone oil droplet was split into smaller droplets and one of them was delivered to the BM-electrode by means of the DEP forces by electrical actuation on the device. Figure 4 is the optofluidic response of the BM-electrode showing that the resonant peak of the BM experienced the same redshift (i.e., 15 nm) as that found when dripping the silicone oil by a pipette (cf Figure 2). This evidence proved the successful integration of an optofluidic BM transducer onto the EMF device and the successful driving of liquid droplets by DEP actuation. These results showed that the porous BM has worked acceptably well in combination with the EMF optofluidic device and we attributed the lesser shift observed in comparison with the reference BM to some microstructural changes induced by the growth of the porous thin films on the Teflon coating covering the ITO layer of the EMF device.

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