and decryption of multiple levels of information. This study opens new avenues for
multilevel imaging, data recording, and security protection of fingerprint information
with tunable fluorescent hydrogels.
3.2.4 Electrostatic Interactions
The dynamic ionic cross-links occur via the reversible electrostatic interactions
between a charged cross-linked polymer chain and an oppositely charged polymer
chain or oppositely charged ions, which introduce the smart behavior on natural
biopolymer hydrogels [26, 187, 188], such as environmental sensitivity and selfhealing behavior. Particularly, there has been significant attention on dynamic
cellulose-based hydrogels cross-linked by the electrostatic interactions.
Chang et al. [189] synthesized a novel ampholytic hydrogel with pH and saltresponsive properties by cross-linking quaternized cellulose (QC) and
carboxymethyl cellulose (CMC) with epichlorohydrin (ECH) in NaOH aqueous
solution, where QC and CMC were employed as the polycations and polyanions,
respectively. In addition to the chemical cross-links, the electrostatic interactions
formed by the acidic (–COO
À ) and basic groups (–(CH 3 ) 3 N
+
) within the obtained
polysaccharide hydrogel network led to the multiple responsive behaviors, including
pH and salt. The results revealed that CMC mainly contributed to the increasing
swelling as a result of strong water adsorption, whereas QC played a domain role in
pH sensitivity by controlling the charges in the QC/CMC system.
Subsequently, Chang’s group [190] reported an interface compatible
nanocomposite hydrogel prepared by introducing quaternized tunicate cellulose
nanocrystals (Q-TCNCs) into chemically cross-linked poly(acrylic acid) (PAA)
networks (Fig. 12a). Notably, the electrostatic interaction between the positive
charges of Q-TCNCs and negative charges of PAA chains improved their interface
compatibility, resulting in improved mechanical strength, toughness, and recoverable ability. Similarly, Chang’s group [192] continued to investigate tough and selfrecoverable hydrogel reinforced by quaternized tunicate cellulose nanocrystals
(Q-TCNCs) in dual physically cross-linked nanocomposite networks. Q-TCNCs
acted as both interfacial compatible reinforcements and multifunctional crosslinking agents to form first loosely cross-linked network through the electrostatic
interactions between –N(CH 3 ) 3
+ on the surface of Q-TCNCs and –COO
À on the side
chains of poly(acrylic acid-co-acrylamide) (PAAAM), whereas the second
compacted cross-linking was built by the formation of ionic coordination between
Fe
3+ and –COO
À of PAAAM. Reversible physical bonds (electrostatic interactions
and coordination bonds) in the dual physically cross-linked networks served as the
reversible sacrificial bonds to effectively dissipate energies, which contributed to the
integration of high tensile strength, high ductility, and high toughness. Importantly,
the nanocomposite hydrogels displayed excellent self-recoverability after immersing
in FeCl 3 aqueous solution. The reversible physical interactions endowed TCNCreinforced hydrogels with excellent mechanical properties and recovery properties,
which provided a universal strategy for construction of tough cellulose-based
hydrogels.
346
C. Shao and J. Yang
multilevel imaging, data recording, and security protection of fingerprint information
with tunable fluorescent hydrogels.
3.2.4 Electrostatic Interactions
The dynamic ionic cross-links occur via the reversible electrostatic interactions
between a charged cross-linked polymer chain and an oppositely charged polymer
chain or oppositely charged ions, which introduce the smart behavior on natural
biopolymer hydrogels [26, 187, 188], such as environmental sensitivity and selfhealing behavior. Particularly, there has been significant attention on dynamic
cellulose-based hydrogels cross-linked by the electrostatic interactions.
Chang et al. [189] synthesized a novel ampholytic hydrogel with pH and saltresponsive properties by cross-linking quaternized cellulose (QC) and
carboxymethyl cellulose (CMC) with epichlorohydrin (ECH) in NaOH aqueous
solution, where QC and CMC were employed as the polycations and polyanions,
respectively. In addition to the chemical cross-links, the electrostatic interactions
formed by the acidic (–COO
À ) and basic groups (–(CH 3 ) 3 N
+
) within the obtained
polysaccharide hydrogel network led to the multiple responsive behaviors, including
pH and salt. The results revealed that CMC mainly contributed to the increasing
swelling as a result of strong water adsorption, whereas QC played a domain role in
pH sensitivity by controlling the charges in the QC/CMC system.
Subsequently, Chang’s group [190] reported an interface compatible
nanocomposite hydrogel prepared by introducing quaternized tunicate cellulose
nanocrystals (Q-TCNCs) into chemically cross-linked poly(acrylic acid) (PAA)
networks (Fig. 12a). Notably, the electrostatic interaction between the positive
charges of Q-TCNCs and negative charges of PAA chains improved their interface
compatibility, resulting in improved mechanical strength, toughness, and recoverable ability. Similarly, Chang’s group [192] continued to investigate tough and selfrecoverable hydrogel reinforced by quaternized tunicate cellulose nanocrystals
(Q-TCNCs) in dual physically cross-linked nanocomposite networks. Q-TCNCs
acted as both interfacial compatible reinforcements and multifunctional crosslinking agents to form first loosely cross-linked network through the electrostatic
interactions between –N(CH 3 ) 3
+ on the surface of Q-TCNCs and –COO
À on the side
chains of poly(acrylic acid-co-acrylamide) (PAAAM), whereas the second
compacted cross-linking was built by the formation of ionic coordination between
Fe
3+ and –COO
À of PAAAM. Reversible physical bonds (electrostatic interactions
and coordination bonds) in the dual physically cross-linked networks served as the
reversible sacrificial bonds to effectively dissipate energies, which contributed to the
integration of high tensile strength, high ductility, and high toughness. Importantly,
the nanocomposite hydrogels displayed excellent self-recoverability after immersing
in FeCl 3 aqueous solution. The reversible physical interactions endowed TCNCreinforced hydrogels with excellent mechanical properties and recovery properties,
which provided a universal strategy for construction of tough cellulose-based
hydrogels.
346
C. Shao and J. Yang