Degree of substitution of chitosan

The degree of substitution of chitosan can be directly measured via solid state 13C NMR

analyses. The test was carried out at spinning rate of 4000 Hz and recording frequency of

50.32 MHz, with 2048 scans at room temperature.

The value was calculated via the following equation [1]:

DS (%) = 100 I (CH3)/ ((I (C1) + I (C2) + I (C3) + I (C4) + I (C5) + I (C6))/6) (1)

where, I is the intensity of resonance peaks.

Figure S1 shows the intensity of CH3 and C1 to C6 peaks, for them DS (%) is 82%:

DS (%) = 100 (57.813) / (65.221 + 75.164 + 76.571 + 25.828 + 126.334 + 43.615)/6)

Figure S1. Solid state 13C NMR on chitosan.

Composite morphology

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Scanning electron microscopy was employed to characterize the morphology of untreated and

freeze-thawed hydrogels. The fracture surface and surface area of samples was coated by

gold and images were taken at 20 kV acceleration voltage.

The visual verification of the chitosan/carbonized cellulose fibres interaction can be seen in

SEM images of chitosan/CC20/FT (Figure S2a) and chitosan/CC30/FT (Figure S2b). The

fracture surface of composite hydrogels shows an embedded network of carbonized cellulose

fibres within the chitosan matrix. Both Figures S2a and S2b are at same magnifications but as

expected Figure S2b shows a larger concentration of the carbonized cellulose fibres. It can

also be observed (figures S2c and d) that, in contrast to chitosan/CC20/FT composite, there

are plenty of fibres that have pulled out or de-bonded from the matrix, in chitosan/CC30/FT.

Therefore, in this study, the sample with 20 wt% of fibres was considered to further

investigations.

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Figure S2. Scanning electron microscopy (SEM) micrographs of (a) chitosan/CC20/FT

(surface area), (b) chitosan/CC30/FT (surface area), (c) chitosan/CC20/FT (fracture surface),

and (d) chitosan/CC30/FT (fracture surface).

Cyclic voltammetry

The cyclic voltammetry technique was used for electrochemical characterization of

hydrogels.

The carbon paste electrodes were prepared as follows: 10 mg crumpled composite sample, 19

µL paraffin and 90 mg graphite powder were uniformly blended. The mixture was packed

into the empty hole of the carbon paste electrode, which was polished to get a smooth surface

and labelled “m-CPE”. Similarly, the bare carbon paste electrode was prepared by only using

paraffin oil and graphite powder and was labelled as “b-CPE”. A cyclic voltammetry test was

conducted at an Autolab electrochemical workstation, with a conventional three-electrode

cell connection. The chitosan based composites were the working electrodes, a Pt mesh acted

as counter electrode and a phosphate buffer solution (PBS) was used as the supporting

electrolyte [2].

Cyclic voltammetry of composite hydrogel (Chitosan/CC20/FT) modified electrode (m-CPE)

is illustrated in Figure S3. In neutral media (pH 7.0), CV shows an oxidation peak at about

0.22 V, whereas in alkaline solution (pH 9.2) the peak slightly shifted to left and slightly

shifted toward right in acidic media (pH 4.5), illustrating that protons are involved in the

electrochemical process of composites. As can be seen, at both acidic and alkaline media, the

charge current declines compare to that of neutral PBS. In acidic pH, the positively charged

deprotonated chitosan leads to an electrostatic repulsion, changing the physical structure of

the composite, which may block the electron transfer within the hydrogel. Also, while

alkaline media did not positively change the reduction oxidation behaviour of the composite,

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the best electrochemical performance was observed at neutral condition, as many biological

systems work in nearly neutral pH [2].

Figure S3. Cyclic voltammetry performance of Chitosan/CC20/FT in neutral, alkaline, and

acidic pH.

1. Ottey, M.H.; Vårum, K.M.; Smidsrød, O. Compositional heterogeneity of heterogeneously deacetylated chitosans. Carbohydrate Polymers 1996, 29, 17-24.

2. Liu, Y.; Peng, X.; Ye, H.; Xu, J.; Chen, F. Fabrication and properties of conductive chitosan/polypyrrole composite fibers. Polymer-Plastics Technology and Engineering 2015, 54, 411-415.

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