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International Journal of Bioprinting Bacteriorhodopsin-embedded hydrogel device

Figure 3. Photoelectric response of bacteriorhodopsin (br)-embedded hydrogel construct. (A) The composite square-differential photoelectrical response
of hydrogel construct containing br. A constant photocurrent and a differential response were observed when the construct was illuminated by white LED
with a wavelength span of 420–700 nm. (B) The square-wave photoelectric response of a hydrogel construct containing no br. The square-wave response
originated from the indium tin oxide (ITO) glass substrate under the illumination of white LED. (C) The pure differential photoelectrical response of the
br-embedded hydrogel construct under pulsing light with 1 Hz frequency from a 543 nm laser. No square-wave photoelectrical response was observed.

wave response of ITO glass due to the heating effect of nearly all br molecules contribute to the photoelectric
its blue light component. To validate this hypothesis, a response as they are all attached to the working electrode.
543 nm green laser, containing no blue light component Additionally, in the br-embedded hydrogel, protonation
and having significantly less heat effect, was utilized for and deprotonation occur within the hydrogel environment,
the photoelectrical characterization, and pure differential while in a br film-based photovoltaic cell, these processes
photoelectrical response was observed (Figure 3C). These take place in the liquid aqueous environment. Since proton
results explained the composite photoelectrical response mobility is restricted in the hydrogel matrix, this could also
of the br-embedded hydrogel construct immobilized on lead to a reduction in the photocurrent of br-embedded
the ITO glass substrate under the illumination of a white hydrogel. Consequently, the photocurrent generated by the
LED light source. Therefore, the 543 nm green laser light br-embedded hydrogel in a three-electrode photovoltaic
source was used in the pattern recognition experiment as it setup is notably lower than the photocurrent produced by
introduces no observable artifacts.
the br film under the same light intensity.
The differential response, which refers to the response
of the material to changes in light intensity rather than the In addition to the amperometric i-t mode, linear sweep
light itself, is a critical characteristic of the photoelectrical voltammetry and open circuit potential-time mode were
response of br. This phenomenon is commonly observed employed to further characterize the photoelectrochemical
in devices incorporating br films as photosensitive properties of the br-embedded hydrogel. In the linear
elements. 21,26,55,67 When br is immobilized as thin films sweep voltammetry test, a 5 Hz pulsing laser served as the
on an electrode, exposure to light leads to the transfer of light source, and data was recorded both with and without
protons to the electrode, resulting in the generation of a illumination. The difference between the two linear sweep
potential difference between the electrodes. This potential voltammetry curves is illustrated in Figure 4A, displaying
difference gradually diminishes as it drives the electron an alternating photocurrent under illumination. The open
movement, thereby causing the differential response. The circuit potential-time analysis (Figure 4B) displayed a
68
photovoltaic mechanism of the br molecule in the hydrogel differential response pattern similar to the photocurrent
is thus very similar, if not identical, to that of the br molecule curve in Figure 3A, where an initial upward peak was noted
in a solid film, where the br molecule facilitates proton at the onset of illumination, followed by a subsequent
transfer between the working electrode and the electrolyte. downward peak after the illumination period.
Nevertheless, there are several distinctions between br-
embedded hydrogel and br films. For instance, in a three- The relationship between light intensity and the
electrode photovoltaic cell configuration in this study, the amplitude of photocurrent is illustrated in Figure 4C. The
br molecules embedded in the hydrogel can only produce photocurrent increased as the light intensity increased,
detectable photocurrent when they are closely attached to and a linear correlation was observed as presented in the
the working electrode, whereas in a br film-based device, following equation:

Volume 10 Issue 6 (2024) 522 doi: 10.36922/ijb.4454