Further Exploration
a) Comparing the system with biofilm and without biofilm Figure 2 and Figure 3
Before we tested the system with biofilm-anchored CdS nanorods, we tested ones with freely-flowing CdS nanorods. The result is shown in Figure3.
During lighting period, the hydrogen production increases, until we shut off the light at points that correspond to the tips. The curve then goes downward, showing that the hydrogen concentration is lowered, an evidence of bidirectional catalytic activity of hydrogenase. The tip reached 30mV compared to the system with biofilm-anchored hydrogenase, 8mV. This lower catalytic efficiency is possibly due to the lower amount of nanorods added to the system in Figure2, since the binding of CdS to biofilm is a reaction that does not guarantee all the binding. It is also likely that the size of the microspheres, 25 um in diameter, on which the biofilm grow is too small for sufficient binding of all the nanorods. In addition, since the normal size of bacteria is 2 um, we therefore think spheres of bigger sizes should lead to a higher efficiency. However, this seemingly low efficiency system has beat the rate of hydrogen production of previous report this year. The calculation is in the next section.
Figure 3 Hydrogen evolution curve with free-flowing nanorods.
Another point to note between our two systems is that in the process of hydrogen generation without biofilm-anchored CdS, a stir bar with a necessary speed of 800 RPM was needed. But in Figure 2, the system with biofilm, a stir bar was not used. It is likely because the aggregates of NR have a bigger chance in colliding with
E. coli to transfer electrons.
b) Calculating the hydrogen evolution rate of our integrated system.
We are particularly interested in learning what our efficiency is compared to one study reported this year. See reference 1. In calculating the efficiency, we chose the data from the first hydrogen production period. We converted the data in mV into umol/L. The standard curve is provided by the lab who supervised our assay apparatus.
Figure Standard Relationship between voltage data and concentration.
Thus, we obtain the rate of hydrogen evolution: the tip of the first period is 7.061 mV at 500s. This corresponds to 2.179 (0.3086*7.061) umol/L at 500s. Thus the rate is 0.0126 (2.179/500*3mL*1000) umol/s, for 0.1g E. Coli. In comparison with the rate from reference 1, 0.0086mol umol/s. This 46% increase in the efficiency shows that our system not only works, but is also a progress for the study of artificial hydrogen production system.