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Page 4 of 12 Luo et al. Soft Sci 2024;4:7 https://dx.doi.org/10.20517/ss.2023.40

plate electrode was used as the counter electrode, and an Ag/AgCl electrode was used as the reference
electrode. A two-electrode flexible device composed of the synthesized ACO on Ga-LM as the working
electrode and carbon black coated on the surface of another batch of isolated Ga-LM as the counter
electrode was fabricated. The sensitivity can be calculated as the ratio of the slope to the electrode area,
Sensitivity = photocurrent/concentration/area.

Characterizations
The microstructure of ACO and CuO -Ga electrodes were observed using scanning electron microscopy
x
(SEM, S-4800) with energy-dispersive X-ray spectroscopy (EDS, Oxford). The Raman spectra and
photoluminescence spectra of the ACO electrode were measured at room temperature and excitation
wavelength of 532 nm using a WITec-Alpha 300 Raman microscope. The crystal structure and
crystallization of the sample were studied using an X-ray diffractometer (XRD-D8 Discover). X-ray
photoelectron spectroscopy (XPS) was used in combination with one-chamber electron spectroscopy for
chemical analysis (ESCA) to analyze the oxides in these samples.

RESULTS AND DISCUSSION
The ACO electrode was fabricated through a simple printing process of Ga-LM onto the surface of ITO
glass, followed by in-situ growth of cuprous oxide on the Ga surface and the final annealing treatment
[Figure 1]. Specifically, when the printed Ga-LM electrode came into contact with oxygen, a thin oxide layer
was immediately formed on its surface. To remove the surface oxide layer and expose the Ga surface, the
[28]
electrode was washed with a base [12,27] . Subsequently, galvanic replacement reactions were carried out
between [Cu(OH) ] and Ga on the electrode surface, and the final stable ACO layer was formed after
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3
annealing.
The morphology of the as-prepared ACO electrode was characterized with SEM, and the resulting mapping
images are presented in Figure 2A-C. The SEM characterization results showed a flat surface with several
ripples was formed, as depicted in Figure 2A . The inset of Figure 2A shows a high-magnification view of a
[29]
randomly selected area on the flat ACO surface, and this image clarifies that the flat surface was composed
of interconnected nanoparticles. In comparison, as shown in Supplementary Figure 1, while the surface
morphology of the CuO -Ga electrode is similar, its surface is slightly less smooth, and a few defects exist
x
between the small copper oxide particles. This implies that after annealing treatment, the consistency of the
surface layer increased, and interfacial contact between the ACO film and the LM substrate improved. EDS
mapping of the ACO electrode shows the relatively homogeneous distribution of elemental Ga, O, Cu, and
C throughout the duration of the galvanic reaction and annealing treatment [Figure 2B]. The atomic
percentages of Ga, Cu, C, and O were around 85.6%, 1.14%, 6.2%, and 7.06%, respectively, as extracted from
the energy spectrum shown in Figure 2C. Owing to the fluidity of the LM, flexible PE, paper, and PE
terephthalate (PET) can be used as suitable substrates for printing LM with a uniform distribution
[Figure 2D]. After the solution containing [Cu(OH) ] was added and the galvanic reaction was triggered,
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the color of the electrodes printed on different substrates changed from white to red [Figure 2E].
Accordingly, in addition to preparing ACO electrodes on ITO glass for use in the fabrication of glucose
sensor devices, flexible integrated devices can be fabricated using the aforementioned flexible substrates.
Figure 2F depicts a flexible integrated glucose sensor device fabricated by printing an LM pattern on a PE
substrate. This device is composed of a concentric-ring-shaped two-electrode system, where ACO on
Ga-LM is the working electrode, and carbon black on Ga-LM is the counter electrode. The device has good
flexibility, and it can be bent to various angles [Figure 2G and H].