Fig. 1 shows the SEM images of the nanocellulose samples
with different degree of DNA-immobilization. In the non-coated
nanocellulose sample a fibrous structure is clearly visible as previously described [43]. It is seen from this graph that as the
amount of immobilized DNA increases, new structures progressively emerge in the form of patches which cover the voids between
the nanofibers. At the highest degree of DNA binding these patches
become denser and locally clog the pore structure. Although it has
been commonly conceived that DNA shows high affinity to cellulose, the images presented here show that DNA does not evenly
coat the cellulose nanofibers but instead forms patches and islands
on nanocellulose. This conclusion was further verified by AFM as
presented in Fig. 2.
Fig. 2 shows the AFM images of the samples coated with varying
amounts of DNA. It is seen from these images that as the amount
of DNA coating is increased the fibrous texture of nanocellulose
becomes locally non-porous.
Table 1 summarizes the results of the phosphorus element analysis of the studied samples. It is seen from table that the amount
of P is progressively increasing as the amount of added DNA is
increased, suggesting that DNA is indeed bound to cellulose.
The strength of DNA binding to cellulose was tested in two different media, viz. in distilled water and in 1 M hydrochloric acid.
Prior to release tests the samples have been thoroughly washed
with distilled water at room temperature to remove the excess
non-bound DNA.
As it is seen in Fig. 3, DNA was firmly bound to cellulose, and
only a small amount of it was released in distilled water from the
samples with the highest degree of DNA immobilization, i.e. 20 mg
DNA/g cellulose, depending on the temperature of the medium.
For the 10 mg DNA/g cellulose, the extent of leakage was within
the error range at the sensitivity limit of the instrument, and the
effect of temperature was inconclusive. No signs of leakage were
detected for DNA loading