Q37. Based on the analysis of the data you summarized in Table 6, how does particle size affect the molar extinction coefficient?
In general, the molar extinction coefficient increases with increasing particle size.
Q38. How does the citrate to tetrachloroauric acid ratio affect the molar extinction coefficient?
Since the citrate to tetrachloroauric acid ratio affects the particle size, the ratio does affect the molar extinction coefficient. As the particle size increases, so does the molar extinction coefficient.
Q39. Does pH play a role in the value of the molar extinction coefficient?
In the pH range explored in this experiment, pH does not dramatically affect particle size and, therefore, molar extinction coefficient. However, for pH values lower than 4, pH plays a dramatic effect on particle size, thus affecting the molar absorptivity coefficient.
Q40. If you were to develop a colorimetric sensor, what experimental conditions would you choose to synthesize nanoparticles of consistent size with a large molar extinction coefficient?
This is an open ended question. Looking at the overall results, particles with an approximate diameter of 20 nm can be synthesized with relatively small polydispersion using a 2:1 citrate to auric acid ratio at pH values between 4 and 5. By increasing the citrate to auric acid ratio to 4:1, the particle size increases along with increased polydispersion. This trend continues for the higher ratio (7:1) where particle size is above 30 nm with wider particle distribution.
For sensor application, probably the 2:1 citrate to auric acid at pH values between 4 and 5 would yield the best conditions.