Libro: “Avenida Brasil Curso Básico de Português para estrangeiros” E.P.U (1992) 175 Páginas.

It is clear from figure 7-5 (a) that there are no steps for non-sensitized optical fibre (green curve, control experiment), hence no observable interaction between Zn(P- CO2H-TPP) film and the octylamine. This is a confirmation that indeed Zn(P-CO2H-

TPP) molecules in the film coated on the stripped section of the optical fibre are responsible for detecting octylamine. On the other hand, there is a larger change in lock-in amplifier output voltage (mV) from experiment with roughened than from “smooth” optical fibres. The arrows on the curve for roughened optical fibre (red) in figure 7-5 (a) indicate the 5 μM aliquot of octylamine titrated at each step. Such steps are also visible in the raw data for “smooth” optical fibre though not shown with arrows and they indicate the interaction of octylamine with Zn(P-CO2H-TPP) film coated on

the optical fibre. The “flattening off” of the curve at each step indicates the equilibrium being reached between the diffusing octylamine molecules into the Zn(P-CO2H-TPP)

film and those of Zn(P-CO2H-TPP). This is controlled by the equilibrium constant, K

(K in Langmuir-Freundlich adsorption model discussed extensively earlier in chapter 2, sections 2.6.3).

Again, we followed a similar procedure described earlier in chapter 5 section 5.3.5.1 to calculate the change in lock-in amplifier output voltage, ΔV (mV) from figure 7-5 (a) and plotted it against the concentration of octylamine (µM) to obtain calibration curves in figure 7-5 (b) (dotted lines, red and black are results for smooth and rough optical fibres respectively). To characterize the adsorption of octylamine molecules into the adsorption sites in the Zn(P-CO2H-TPP) film, we fitted the experimental data using

origin 2017 software. After initial attempts to use the Langmuir and Freundlich adsorption models separately, no good fits to the experimental data were realized, (see poor fits in appendix XV for linear form of Langmuir fit as an example). However, the Langmuir-Freundlich adsorption model gave good fits to the experimental data as shown in figure 7-5 (b) (continuous curves, red and black for smooth and rough optical fibres respectively). The fit parameters are shown in the table inset figure 7-5 (b) and can be seen clearly that the correlation coefficients, r2 _{= 0.991 and 0.989 for smooth}

and rough optical fibres respectively (see table 6). This indeed confirmed that the experimental data fit into Langmuir-Freundlich adsorption model.

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(a)

(b)

Figure 7-5. Comparison of the performance of roughened and “smooth” optical fibres. (a) raw data indicating the adsorption of octylamine molecules into the adsorption sites in Zn(P-CO2H-TPP) thin film coated on the exposed core of the optical fibre. Red, blue and green are the data measured for experiment carried out with rough, smooth and non-sensitized optical fibres respectively. Here arrows indicate titration of 5 μM octylamine (b) calibration curves (5% error) for octylamine sensing derived from raw data in figure 7-5 (a), (red dots- smooth fibre, black squares -rough fibre while black and red continuous line are Langmuir-Freundlich fits for rough and smooth fibre respectively).

The fitted data from Langmuir-Freundlich model are summarized in table 6 and we know [147] from this model that; n is an index of homogeneity, K is the equilibrium constant and ΔVsat relates to the total number of binding sites available on the Zn(P- CO2H-TPP) film coated on the optical fibre. This means when all the binding sites in

Blank 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 250 500 750 1000 1250 Lo ck -i n V o u t (m V ) Time (seconds)

Rough fibre optic "Smooth" fibre optic Control experiment

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the Zn(P-CO2H-TPP) film have been occupied by octylamine molecules, the change in

lock-in output voltage is equal to ΔVsat.

To find the LoD and sensitivity, we followed similar procedure as reported by [188, 189] by selecting the linear range of a calibration curve near zero concentration and fitting a linear model. Therefore, the linear sections in the calibration curves in figure 7-5 (b) fall between 5 µM and 37.5 µM and using origin 2017 software, we fitted linear regression model to the data within this range. The fit data are shown in the table inset in figure 7-6 and the evaluated slope; m (in μV/μM), intercept b (in μV) and the error in the intercept, Δb (in μV) are given. Here, m (in μV/μM) quantifies the sensitivity of the EWS amine sensor. We evaluated the LoD from 3 times the error in y-intercept/ slope as discussed earlier in chapter 2, section 2.2.1 and the results are shown in table 6.

Figure 7-6. Fitted calibration curve (5% error) within the linear range for determination of the LoD of octylamine dissolved in water. Black and red dots are the measured data from rough and smooth optical fibre respectively while black and red dotted lines are the linear fits for measured data for rough and smooth optical fibres respectively.

Table 6: Parameters describing the adsorption of octylamnie molecules onto the binding sites in Zn(P- CO2H-TPP) film coated on both smooth and rough optical fibres.

Parameter ΔVsat (µV) m (µV/ µM) K (µM-1) LoD (µM) Index r2 Rough optical fibre 672.76 13.3249 0.0406 2.17 2.042 0.991 Smooth optical fibre 657.08 7.78759 0.0235 2.44 1.322 0.989

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The quantitative data in table 6 show the sensitivity for roughened optical fibre (13.3249 µV/ µM) is nearly twice that of the “smooth” (7.78759 µV/ µM) optical fibre, which confirms that the evanescent wave intensity on the roughened surface is enhanced compared to that on the “smooth” surface. The equilibrium constant K lies within the same range (0.0406 µM-1 _{for rough, 0.0235 µM}-1 _{for “smooth” optical}

fibres) and we can conclude that the binding of octylamine molecules is not affected by roughening the optical fibre. In addition, there is no improvement on LoD, which means roughening the optical fibre has no significant improvement on the transducer signal to noise ratio. Comparing the number of available active sites for rough and smooth optical fibres from ΔVsat, a difference of 15.68 µV is observed. This means roughening the optical fibres extends the surface area in which the sensitizer film is coated, hence extending the analytical range of the sensor. The index of heterogeneity, taking experimental errors into consideration, is within the acceptable range 0 ≤ n ≤ 1. Finally, ΔG0 = -36244 Jmol-1 and -34890 Jmol-1 at 25 0C for roughened and “smooth” optical fibres respectively (after changing µM-1 into M-1 and using Eq 2.33, chapter 2, section 2.6.4).