The basic experiment for the optical fibre cantilever pH measurement system shown in Figure 4.18. The analysis of the interferogram is based upon extracting the free spectral range of the cavity, which is directly related to the cavity length. SM-125 optical interrogator (Micron Optics, USA) is used to demodulate the phase information from the cantilever. This was already discussed in Chapter 3. Figure 4.19 shows the fringes from the machined cantilever. The cavity length calculated by FFT is ~59.3μm in this case.
Figure 4.18 Schematic of pH measurement set-up. A fibre-top cantilever is immersed into the liquid cell and a thermocouple monitors the temperature. SM -125 optical interrogator demodulates the cavity length and the deflection of cantilever is calculated by LabView/PC. The zoomed in part shows the two reflecting surfaces which create the FP cavity. (a) (b) Syringe pump Liquid cell Laser to cantilever M icro-machined cantilever SM-125 LabView/ PC Waste Flow in
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Figure 4.19 Fringes of a laser/FIB machined optical fibre-top cantilever with a cavity length of 59.3µm. Prior to using the micro- machined cantilever as a pH indicator, it is calibrated with an actuating fibre end to apply a known deflection. The movement of the actuating fibre is controlled by a calibrated piezoelectric translation stage. The process is repeated several times in order to prove the mechanical robustness of this micro-machined cantilever. A ML10 position sensor (Renishaw, plc. UK) is employed to calibrate the sensor with the detailed calibration process discussed in [4.5].
In order to make the cantilever sensitive to pH one side of the cantilever must be ‘activated’, so that a change in pH results in a different strain on each face of the cantilever. It is this strain differential that causes the cantilever to deflect according to the Stoney equation discussed in Chapter 2. Two layers are investigated in this work (i) Al2O3/Au and (ii) MHA/HDT. Both MHA (16- mercapto- hexadecanoic-1-acid) and
HDT (1,6-hexanedithiol ) were purchased from Sigma-Aldrich.
Before testing the pH sensitivity the background system noise level with the cantilever in a fluid was measured. This is achieved by immersing the cantilever into Phosphate Buffer Solution (PBS) pH 7.0 solutions and monitoring the cantilever position. The experiment was temperature controlled (23˚C±0.2˚C) and insulated in order to maintain a stable temperature environment. In order to investigate the temperature effect on the cantilever deflection, a fibre-top cantilever is immersed into the water and a thermocouple monitors the ambient temperature changes over 200 minutes. The relative cantilever deflection is acquired every 10 seconds over 200 minutes, with results shown
1500 1520 1540 1560 1580 1600 0 500 1000 1500 2000 2500 3000 3500 4000 In ten si ty (a. u .) wavelength(nm)
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in Figure 4.20. Here the maximum cantilever deflection fluctuation is around ±12 nm, with an r.m.s. error of ~±6 nm. From this data we conclude that a temperature variation within a degree does not affect the cantilever deflection within the measured noise level. 0 50 100 150 200 -20 -10 0 10 20 30 40
Measured cavity length Temperature Time(mins) C avi ty len g th fl u ctu ati o n (n m) 22.2 22.3 22.4 22.5 22.6 22.7 22.8 o C
Figure 4.20 Thermal stability of cantilever deflection over 200 minutes. ~20nm cavity length fluctuation is found when temperature changes from 22.3°C to 22.7°C.
In order to investigate the effect of flow turbulence upon the cantilever deflection was monitored while gently injecting PBS pH 7.0 solution into the liquid chamber at a constant speed of 50μL/min. The cantilever is initially immersed in PBS pH 7.0 solution and a thermocouple is also placed in the liquid cell to monitor the temperature change. The cavity length and temperature are shown in Figure 4.20. After injection the temperature dropped down rapidly by 0.3˚C then returned back to 23˚C after 40 seconds. The rapid drop of the temperature was due the lower temperature difference between the injected liquid and the ambient temperature. As the liquid flowed outside the liquid cell, the temperature started to return to ambient temperature. An average (r.m.s) cavity fluctuation during this period was found to be 12.8 nm, which was comparable to the system noise level. Therefore by carefully controlling the injection speed, the influence of flow turbulence on the cantilever deflection can be minimised.
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For pH measurements the cavity length is measured in reference to the initial cavity length when the sensor is immersed into PBS pH 7.0 (defined as 0 nm). We define ‘bending down” as cantilever bending towards the gold coated side while, ‘bending up’ refers to bend towards the Al2O3 side. The experiment is first conducted to test the
thermal stability of the micro- machined cantilever with a coating on it in a liquid environment. The whole experiment is carried out at a temperature of 23˚C±0.2˚C, with the interrogation system shown in Figure 4.18. The system is covered with foam and filled with insulating material in order maintain a stable temperature environment. After that, the cantilever is immersed into PBS 7.0 solution and the relative cantilever deflection is acquired every 10 seconds over 200 minutes (Shown in Figure 4.21). It’s clear that the maximum cantilever deflection fluctuation is around ±12nm, with an r.m.s. error of approximate ±6nm, which means for temperature variation within a degree, the deflection noise level of cantilever is ~8nm, therefore, small temperature changes of <0.5˚C is within the noise level.
Figure 4.21 Cantilever cavity fluctuation versus temperature change over a measurement time of 140 seconds. (a) Fibre-top cantilever deflection before and after injection of flow. (b) Flow speed versus injection time. Injection starts at 40s and finishes at 125s.
(a)
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