pH-Responsive DNA Nanolinker Conjugated Hybrid Materials for Electrochemical Microactuator and Biosensor Applications

Background

Carbon nanotube composites are used for microactuators, sensors and capacitors, but specific functions such as sensing or deformation require stimulus-responsive elements in the material. i-motif DNA, a cytosine-rich strand that folds into a four-stranded structure at acidic pH and relaxes into a random coil at higher pH, offers such a switch. An earlier i-motif DNA-CNT material lacked binders and could not be formed into free-standing films or fibers, and coagulation in ionic liquids, used previously to make porous dsDNA-SWCNT fibers, deforms DNA irreversibly. CNT-polymer fibers also face a trade-off in which higher microporosity lowers electrical conductivity.

Design and approach

Carboxyl-functionalized SWCNTs are dispersed with dsDNA, which acts as a biosurfactant and polymeric binder, and amine-terminated i-motif DNA is covalently coupled to the SWCNT carboxyl groups with EDC/NHS. The dispersion is extruded into a coagulation bath of 0.3 M HNO3 and ethanol (9:1 v/v): protons neutralize the negatively charged DNA-SWCNTs, reduce their electrostatic repulsion and let them self-assemble into nanofibers, which pack into a porous microfiber. Because the i-motif linker is ∼5 nm long in its folded state and approximately 8 nm as a random coil, switching between pH 5 and pH 8 changes how tightly the nanofiber network is held together.

How it was evaluated

The authors examined the dispersion by high-resolution TEM, circular dichroism (CD), cryo-TEM, zeta potential and UV-vis spectroscopy, and the fibers by SEM, TEM, FT-IR and Raman spectroscopy. Swelling and surface morphology were compared at pH 5 and pH 8. Conductivity was measured with a four-probe cell, and cyclic voltammetry was run in phosphate-buffered saline (PBS) at pH 5 and 8. Electrochemical actuation was tested in 2 M NaCl under an isotonic stress of ∼1.4 MPa with a square-wave potential of +0.2 to −1.0 V, in comparison with fibers lacking i-motif DNA. H2O2 sensing was tested with single fibers over 1 nM to 100 μM and 0.5 to 8 mM at both pH values. All measurements were made in vitro without cells or animals.

Key results

The conjugated i-motif DNA kept its pH response: in CD spectra, the positive band at 287 nm shifted to 270 nm and the negative band at 256 nm shifted to 250 nm on going from pH 5 to pH 8. With increasing HNO3, the DNA-SWCNTs aggregated into clusters at 1 μM, formed slack fibrous networks at 10 to 100 μM and assembled into thicker networks at 1 mM. A water bath gave loose networks, whereas the HNO3/ethanol bath produced densely packed fibers about 200 μm in diameter when dry, with a density of 0.65 g cm−3 compared with 1.5 g cm−3 for an earlier static-DNA fiber from the corresponding authors’ group. The fibers consisted of partially aligned nanofiber bundles ∼50 nm in diameter, and FT-IR showed a new phosphoramidate band at 1385 cm−1 after cross-linking.

At pH 5 the fiber surface was smooth and densely packed, whereas at pH 8 it became rougher with many small pores. Dried fibers absorbed water with volume changes of ∼25% at pH 5 and ∼41% at pH 8, and switching between pH 5 and 8 produced a reversible diameter change of ∼12%.

The porous fibers reached ∼27 S/cm, higher than earlier dsDNA-wrapped SWCNT fibers made with ionic liquids. Strong redox peaks appeared at pH 5 (Epa = 0.11 V, Epc = −0.18 V at 100 mV/s) and diminished at pH 8, and the fibers remained stable over 20 continuous CV cycles. Capacitance normalized by SWCNT mass was ∼59.9 F/g at pH 5 and ∼47.8 F/g at pH 8, compared with ∼7 F/g for earlier electrospun DNA/SWCNT mats and ∼30 F/g for typical SWCNT fibers. Under ∼1.4 MPa and 30 s potential steps, fibers with i-motif DNA actuated with ∼0.15% expansion and contraction without creep, whereas fibers without i-motif DNA showed high creep.

In H2O2 sensing at pH 5, the redox peak near 0.1 V decreased, shifted and finally disappeared as H2O2 rose from nM to mM levels, while CD showed that the i-motif DNA stayed folded up to 10 mM H2O2; the authors proposed that H2O2 complexes with DNA phosphate groups and hinders charge transfer. At pH 8, the CV curves did not change up to 100 μM H2O2. At both pH values a new oxidation peak (Epa: 0.35 V) grew with H2O2 in the millimolar range up to 8 mM, and repeated runs were consistent. The authors suggested that peak position and current could be read together to report both pH and H2O2 level.

How it connects to other research

Chang and colleagues conjugated i-motif and G-rich DNA to SWCNTs and reported redox activity that switched reversibly with pH (5 and 8) and with Li+ and K+ ions; the present study adds dsDNA as a binder so that the i-motif-linked SWCNTs can be wet-spun into free-standing fibers for actuation and sensing.