Electrochemical Performance Adjustion Of Polymer/graphite Oxide-based Electrochemical (bio)sensor

In the field of electrochemical（bio）sensor, the electrode material plays a vital rolein realizing high efficient, high-performance detection of target molecules. And now the selectivity and sensitivity of material-based biosensor have been greatly improved.With the deep-going research in electrochemical（bio）sensor, single material modified electrode can`t meet scientists` requirements, they have put forward new and higher requirements on electrode materials. Functionalization, composite effect, especially synergies effect of composite materials with a view to obtaining a better, more sophisticated electrochemical device, is the goal of electrochemical researchers.Nano-composite materials consisting of polymers and carbon nanomaterials are widely used in electrochemical（bio）sensor. The integration of polymers with carbon nanomaterials may not only generate hybrid systems that combine their own properties, but also obtain new catalytic, photonic and electronic properties, which don`t exist in its single component. Stimuli-responsive polymers used in electrochemical sensing can obtain distinctive detection effects（eg. tunable and switchable electrochemical effect）, which traditional materials don`t have. The application of stimuli-responsive polymers in electrochemical（bio）sensor realize electrochemical detection of intellectualization.In this paper, we studied the performance of the electrochemical（biological）sensor from following aspects: composite effect, the signal response and the change of material structure. And it includes the following four sections:（1） Poly（N-isopropylacrylamide）101（PNIPAM101） was synthesized via visible light activating RAFT polymerization at room temperature utilizing EDMAT as a chain transfer agent, TPO as a highly efficiency photoinitiator and N-isopropylacrylamide as reactive monomer. Poly（N-isopropylacrylamide）101-b-poly（acrylamidoethyl benzoate）37(PNIPAM₁₀₁-b-PAAEB₃₇) was obtained by a chain-extending RAFT copolymerization of PNIPAM101 with acrylamidoethyl benzoate under a visible light radiation. PNIPAM101 act as macromolecule chain transfer agent. GPC and 1H NMR analysis of PNIPAM101 and PNIPAM₁₀₁-b-PAAEB₃₇ confirmed they have intact molecular structure, narrow distribution and well-defined molecular weight.Transmittance of PNIPAM₁₀₁-b-PAAEB₃₇ solution at different temperature via UV-vis spectrum showed it has temperature-responsive phase transformation behavior with LCST of ³2 ℃.（2） Using PNIPAM₁₀₁-b-PAAEB₃₇ and graphite oxide（GO） composite film as a support for hemoglobin（Hb） immobilization constructed a temperature-responsive biosensor. This composite film provided a friendly microenvironment for Hb to transfer electrons with underlying electrode. Hb in this film displayed a couple of well-defined and stable redox peaks, and exhibited intrinsic electro-catalytic activity toward H₂O₂. Because of the existing of temperature-responsive polymer in this film,Hb showed temperature-tunable electrochemical behaviors.EIS measurement of PNIPAM₁₀₁-b-PAAEB₃₇/GO/Hb（PGH） film showed that it behaved as a temperature-controlled interfacial resistance switch. Upon temperature stimuli from 26 ℃ to 36 ℃, Hb displays two different cathodic current states. A high cathodic peak current is observed in shrunken state of PNIPAM₁₀₁-b-PAAEB₃₇ above32 °C and a low one is observed in the swollen state below 32 ℃. Also Hb exhibited two different catalytic activity states toward H₂O₂, above 32 ℃, the catalytic activity was higher than below 32 ℃.The temperature-tunable catalytic activity is attributed to the "expansionto-contraction" transition of PGH film induced by phase transition of PNIPAM₁₀₁-b-PAAEB₃₇ upon temperature change. The state change of composite membrane caused the difference of Hb micro-environment, and then lead to different direct electrochemical properties and catalytic activity of Hb above and below 32 °C.（3） A temperature-responsive composite sensing film（PGS film） consisting of PNIPAM₁₀₁-b-PAAEB₃₇, graphene oxides（GO） and short multi-walled carbon nanotubes（SMWCNT） were fabricated as working electrode, and this as-developed sensing film showed reversible and switchable properties toward temperature stimuli.Reversible voltammetry peak pairs of hydroquinone（HQ） and catechol（CC） with large peak currents at PGS modified electrode were observed above LCST of PNIPAM₁₀₁-b-PAAEB₃₇, and then disappeared below LCST. This indicated that the composite sensing film presented excellent temperature-controlled “on/off” detection effect toward HQ/CC. At 36 °C, HQ/CC shows a quite reversible CV peak pair with relatively large peak current, indicating that the PGS film is at “on” state; at 26 °C, the CV response is significantly reduced and even hardly be observed, the film is at “off”state.Comparing with the “on/off” effect of the reported switchable electrodes modified only by temperature-responsive polymers, an opposite electrochemical “on/off”behavior was notice, which can be attributed to temperature-dependent phasetransition behaviors of PNIPAM₁₀₁-b-PAAEB₃₇ and synergistic effect of the other two functional components（SMWCNT and GO）. Repeatable “on/off” voltammetry responses of HQ/CC at PGS electrode were achieved through controlling the solution temperature.（4）（POPHPMA21-r-PEGMA31）-b-POPHPMA10（POEO） has good biocompatibility, favourable penetrability, but bad conductivity. GO belongs to a member of carbon nanomaterials, and exhibits good hydrophilicity, a moderate conductivity, and poor penetrability. These two materials show inverse properties. We mixed the two material at different ratios for Hb immobilization, and compared the electrochemical behavior and electrochemical catalytic activity toward H₂O₂ and NO, and further studied the factors of influencing Hb electrochemical properties.With the increase of GO ratio in polymer/GO composite film, the CV peak currents of Hb begin to increase and then decrease, but the electrochemical catalytic activities of Hb toward NO and H₂O₂ are just contrary, it begin to decrease and then increase. The highest CV peak currents and the worst electrochemical catalytic activity all appear at 3/7 ratio of POEO/GO. This result indicate that the fators of influencing Hb direct electrochemical properties are biocompatibility and conductivity of composite biosensing film. But the electrochemical catalysis of Hb toward H₂O₂ and NO belongs to membrane outside detection. In addition to consider the two above-mentioned factors, the permeability of composite film should be also considered.（5） We used GO or r GO to directly modify glassy carbon electrodes, they all show good catalytic activities toward HQ/CC and can be used to detect HQ/CC. This two carbon nano-material based electrochemical sensor has a simple assembly of electrode, high sensitivity, fast signal response, wide linear detection range, good stability, and so on. Because of their structure difference, their catalytic activities toward HQ/CC are different. HQ/CC displayed a high peak current in r GO electrode than GO electrode. So r GO based electrode showed a better sensitivity toward HQ/CC.This result showed r GO electronic conductivity is more conducive to good detection of hydroquinone. It also explains the conductivity of electrode material plays an important role on the substrate detecting.