3. Results
3.4 SERS analysis of APIs on cysteamine-based chemical sensors
3.4.3 Lamivudine on cysteamine-based sensors
99 Table 3.12:Statistical values of TDF on glass, Au@Cys/Au and Ag@Cys/Ag.
According to the table above, the chemical sensors showed a trend in analytical sensitivity towards the adenine ring as follows: the glass gave the highest value of 39.26 followed by silver with 26.36 and lastly gold produced a low value of 16. This trend can arise by considering the standard deviation of the data from glass decreased relative to the slope or calibration sensitivity whose linearity is confirmed by the R2 value. Since the latter was low in magnitude, implying a poor linear fitting, the analytical sensitivity value received is not considered reliable. When comparing the nanomaterial sensors, silver surpassed the gold value due to the high calibration sensitivity and lower standard deviation. The limit of detection from glass was found to be 0.05 mg/ml, while lower values were obtained from gold at 0.01 mg/ml and silver and 0.003 mg/ml respectively. In the case of the amine bonds of the ring at 1310 cm-1, the analytical sensitivity of glass was higher than gold at 43.13 and 20.6 respectively, while the silver sensor held the highest value at 52.24. For this functional group, the R2 value on glass improved from the adenine ring.
However, it remained lower than the chemical sensors with an additional lower significance value and thus it was regarded unreliable. The limit of detection significantly dropped on the chemical sensors compared to the glass substrate, with silver producing the lowest value of 0.001 mg/ml and gold at 0.01 mg/ml. The overall observation from the table above is that silver produced the most optimum combination of parameter values against gold and glass. In the next section, experiments with Lamivudine were conducted to compare the performance of the chemical sensors against the glass.
100 gold and silver chemical sensors because the glass samples were already explored and discussed in the citrate-based section of 3.4
Figure 3.33: Raman spectral overlay of Lamivudine on Au@Cys/Au, 0.001-10 mg/ml. Acquired from 200-1800 cm-1.
532 nm, 12 mW laser power, 100 scans, 10 seconds per scan. Insert: LAM molecular structure.
Samples of Lamivudine were run on the Raman microscope system to analyze the molecular properties of the Au@Cys/Au chemical sensor. The blue line in the spectral overlay shows the Au@/Cys/Au contribution from section 3.2 which was background subtracted. The succeeding spectral lines are from the LAM samples. The bands observed from the samples showed the amine bending mode red-shifted by 5 cm-1 at 222 cm-1 followed by the combination of the twisting mode of carbon sulphur bonds of ring 1 and the bending mode of the hydroxyl group (R1) at 463 cm-1 . The molecular skeleton is seen twisting in two bands 595 cm-1 and 628 cm-1 which are slight blue shifts of less than 4 cm-1. In the 800 cm-1 regions a doublet peak was detected and assigned to deformation modes of R1 and R2 both red-shifted, implying the rings were expanding under tensile strength. Carbon oxygen bonds from R1 were detected at 1032 cm-
1. The rest of the spectral range was dominated by R2, with the amine bonds in bending mode and carbon nitrogen bonds seen wagging and stretching at 1246 cm-1, 1292 cm-1 and 1525 cm-1 respectively. At the end of the spectral range, amine bands and aromatic carbon bonds are detected in combined stretching and deformation modes in the 1600 cm-1 region, with the carbonyl carbon bond in stretching mode at 1643 cm-1. Thus, as seen in the citrate study, the R1 seems to dominate the signal response via the aromatic character and the amine groups again in this sample group and as such, the same bands of interest selected in 3.4.3 were chosen for
101 comparison and statistical analysis with cysteamine. The next figure is a spectral overlay of LAM on Ag@Cys/Ag sensor.
Figure 3.34:Raman spectral overlay of Lamivudine on Ag@Cys/Ag, 0.001-10 mg/ml. Acquired from 200-1800 cm-1.
532 nm, 12 mW laser power, 100 scans, 10 seconds per scan. Insert: LAM molecular structure.
The Raman spectral overlay in figure 3.34 above shows the response of LAM on Ag@Cys/Ag sensor. The amine bending mode was again detected in bending mode at 226 cm-1 which is closer to the powder value compared to gold. R1 thiol and hydroxyl groups were seen at 463 cm-1 twisting modes alongside the skeletal bands in 600 cm-1 also in twisting mode [58-59]. The doublet seen in figure 33 was absent in this sample group, instead a broad peak in the 800 cm-1 region was detected arising from the vibrational bands of the two rings in deformation mode.
The C-O bond of R2 was again detected at 1032 cm-1. A blueshift character is seen on the rest of the spectrum with R2 bands, C-N and the amine group appearing in bending mode in the 1200- 1300 cm-1 region followed by the deformation of the amine group coupled with the imine stretching vibration at 1525 cm-1. In addition, the amine group deformation reappeared again coupled with the carbon double bonds of the R1. This implies most of the vibration bands in this region were in a compressed state which indicates bond shortening of the molecule in response to the chemical sensor and laser photons [60]. Lastly, the stretching vibration of the carbonyl group was seen at 1640 cm-1 at the end of the spectral range.
102 Table 3.13:Raman bands of Lamivudine on glass, gold and silver sensors.
ν=stretching, δ=rocking, ρ=bending (in plane),ω= bending (out of plane), τ= twisting, [33].
The peak areas of 1249 cm-1 and 1292 cm-1 for both gold and silver were used in the next section to perform statistical analysis on the bands in response to the samples in the spectral shown in this section.