L. Santos, J. P. Neto, A. Crespo, P. Barquinha, L. Pereira, R. Martins, E. Fortunato CENIMAT/I3N, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
Motivation Introduction Hydrothermal synthesis of WO 3 WO 3 characterization ph sensor assembly Characterization and proof of concept Conclusions
Advantages: Early diagnosis Continuous monitoring Requirements: Cost-effective Flexible Lightweight Easy-to-fabricate Biocompatible Examples: ph Temperature Pressure Humidity Surrounding gases Segev-Bar, M., et al., ACS Nano 7, 2013, 8366
ph value can be useful as: indicator for diagnosing diseases optimizing medical treatments monitoring biochemical and biological processes Metal oxide sensing layer: Low manufacture costs Compatibility with miniaturization processes High sensitivity Abundancy Potentiometric sensor: Simple Portable electrochemical technique Bandodkar, A. J., et al., Analyst 138, 2013, 123
Semiconductor Band gap 2.7 ev. Applications: electrochromic, gas sensor, electrocatalyst, thermochromic and photochromic 100 nm 200 nm Redox reaction:
NaWO 4 + HCl + NaCl Tungstic acid (H 2 WO 4 ) Synthesis: 1 hour 180 C Nanostructured WO 3
WO 3.0.33H 2 O orthorhombic + Non identified product (*) ReO 3 -type structure with WO 6 octahedra Intensity (a.u.) * * * * WO 3.0.33H 2 O (ICDD 01-072-0199) 10 20 30 40 50 60 70 2 (Degrees) - W - H
Agglomerates of 100 nm with Nanoparticles of 10 nm 100 nm 100 nm 50 Type IV isotherm: Mesoporous material 0 0.0 0.2 0.4 0.6 0.8 1.0 Relative Pressure (P/P 0 ) Volume adsorbed (cm³/g)100 Gas (N 2 ) adsorption Desorption Adsorption
Polyimide substrate 2. E-beam evaporation of Ti/Au 4. Electrodeposition of WO 3 np s 1. Shadow mask with laser cut 3. Wax printing
Polyimide substrate Contact area Connection line 1 mm 20 mm 20 mm 1 mm Printed wax as insulator is cytocompatible* Ti/Au Wax WO 3 np s Sensing area 1 mm 2 *International Standard (ISO 10993-5) using extract method
Constant current deposition: 20 µa for 900 s Drying: 60 C for 1 H
10 6 0 Zmodulus (W) 10 5 10 4 10 3-30 Au -60 Au/ WO 3 Zphase (º) 100 nm 10 2 10 0 10 1 10 2 10 3 10 4 10 5 10 6 Frequency (Hz) Increment of the electrochemical surface area Carbon sacrificial layer WO 3 nanoparticles Ti/Au 50 nm
Au/WO 3 Flexible Electrode Au Flexible Electrode Potential (V vs Ag/AgCl) 0.75 0.70 0.65 0.60 0.55 0.50 0.45 Sensitivity: -56.7 ± 1.3 mv/ ph R 2 = 0.995 3 electrodes 9 8 7 6 5 ph 0.18 0.14 0.10 Potential (V vs Ag/AgCl) 0.22 0.60 Sensitivity: -26.5 ± 1.4 mv/ ph R 2 = 0.989 9 8 7 6 5 ph Theoretical sensitivity of -59 mv/ph according with Nernst equation: E = E 0 (2.303 RT/F) ph = E 0 0.05916 ph
Potential (mv vs Ag/AgCl) 700 600 500 ph=5 ph=6 ph=7 ph=8 ph=9 0 500 1000 1500 2000 2500 3000 Time (s) Some degradation of the interface due to deffects and/or charge trapping
1.0 0.5 Time measured to reach 90% of the potential, starting in air: 23 to 28 seconds Potential (V vs Ag/AgCl) 0.0-0.5 1.0 0.5 0.0-0.5 1.0 0.5 ph 9 ph 7 0.0-0.5 ph 5 0 20 40 60 80 100 120 140 Time (s)
Transmittance measurements @ 280 nm Calibration curve Transmittance (%) 100 50 Transmittance (%) 100 50 y = 98.89 2077.8x r 2 = 0.997 0 2 4 6 8 Time (Days) 0 5 10 15 20 25 [WO 3 ] (mg/ml) In UV-vis spectroscopy and SEM images: No degradation after 8 days in PBS @ 37 C
15.0k Reference electrode production: 1. E-beam Cr/Pt/Ag 2. Anodization in HCl 3. Saturation in KCl 0 Gelatin electrolyte in buffer solution Ag/AgCl reference electrode Zmodulos (W) 10.0k 5.0k -10 Zphase (º) 0.0 After anodization After saturation 10 2 10 3 10 4 Frequency (Hz) -20
305 Sensitivity: -14.5 ± 0.9 mv/ ph R 2 = 0.993 Gelatin electrolyte in buffer solution Potential (mv) 295 285 Ag/AgCl reference electrode 275 8.0 7.0 6.0 ph Instability of the reference electrode Low ion mobility of the solid electrolyte
Sensitivity of the sensor close to the theoretical value ( -59 mv/ph) High reproducibility Good reversibility of the sensor after 3 complete cycles, from ph 9 to 5 and in reverse way Wax is cytocompatible and therefore a good option to insulate the sensor Sensor produced with low cost techniques and with good conformation to curve surfaces Applications from biomedical flexible devices but can also be adapted to other applications such as food packaging, soil monitoring in agriculture, erosion monitoring in construction or even lubricants.
CENIMAT MEON Group 2014 INVISIBLE (ERC-2008-AdG 228144) PhD grants: SFRH/ BD/ 73810/ 2010 and SFRH/BD/76004/2011
ls.santos@campus.fct.unl.pt