Reversible Switched Detection of Dihydroxybenzenes Using a Temperature-sensitive Electrochemical Sensing Film

Thermally responsive reduced graphene oxide with electroactive functionality for controllable electroanalysis

A thermally responsive hybrid poly(NIPAm-b-BVIm[FcCOO])-rGO composed of block co-polymer poly(NIPAm-b-BVImBr), reduced graphene oxide (rGO) and electroactive anions was designed and synthesized to achieve electroactive functionality. It is the polymeric ionic liquids (PILs) segment in the block co-polymer that integrated the three different components into a whole hybrid. Such segment of PILs could not only promote the modification of polar PNIPAm onto the non-polar rGO by cation-π interaction, but also realize the immobilization of ferrocenecarboxylate anion (FcCOO^-) via anion-exchange reaction. The PNIPAm moiety endowed the poly(NIPAm-b-BVIm[FcCOO])-rGO with thermal responsiveness, while the anion moiety provided additional electroactive function. It is noteworthy that the conformational change of PNIPAm segment upon different temperature could reveal or seal the redox probe of FcCOO^-, thereby leading to a controllable expression of electroactivity switching by thermal stimuli. Owing to such regulation on surface property and conformation of PNIPAm segment, the modified electrode exhibited excellent thermally responsive electrocatalysis with reversible 'ON-OFF' effect toward the detection of ascorbic acid (AA), which led to two different catalytic states at the same electrode. The reversible electrocatalytic performance with switching capability of the poly(NIPAm-b-BVIm[FcCOO])-rGO/GCE is expected to have a broad application in the field of intelligent electrochemical sensors and devices.

Multimodal/Multifunctional Nanomaterials in (Bio)electrochemistry: Now and in the Coming Decade

Multifunctional nanomaterials, defined as those able to achieve a combined effect or more than one function through their multiple functionalization or combination with other materials, are gaining increasing attention in the last years in many relevant fields, including cargo targeted delivery, tissue engineering, in vitro and/or in vivo diseases imaging and therapy, as well as in the development of electrochemical (bio)sensors and (bio)sensing strategies with improved performance. This review article aims to provide an updated overview of the important advances and future opportunities exhibited by electrochemical biosensing in connection to multifunctional nanomaterials. Accordingly, representative aspects of recent approaches involving metal, carbon, and silica-based multifunctional nanomaterials are selected and critically discussed, as they are the most widely used multifunctional nanomaterials imparting unique capabilities in (bio)electroanalysis. A brief overview of the main remaining challenges and future perspectives in the field is also provided.

Thermoresponsive Diblock Copolymer Films with a Linear Shrinkage Behavior and Its Potential Application in Temperature Sensors

The linear shrinkage behavior in thermoresponsive diblock copolymer films and its potential application in temperature sensors are investigated. The copolymer is composed of two thermoresponsive blocks with different transition temperatures (TTs): di(ethylene glycol) methyl ether methacrylate (MEO₂MA; TT₁ = 25 °C) and poly(ethylene glycol) methyl ether methacrylate (OEGMA₃₀₀; TT₂ = 60 °C) with a molar ratio of 1:1. Aqueous solutions of PMEO₂MA-b-POEGMA₃₀₀ show a three-stage transition upon heating as seen with optical transmittance and small-angle X-ray scattering: dissolution (T < TT₁), self-assembled micelles with core-shell structure (TT₁ < T < TT₂), and aggregation of collapsed micelles (T > TT₂). Due to the restrictions in the polymer chain arrangement introduced by the solid Si substrate, spin-coated PMEO₂MA-b-POEGMA₃₀₀ films exhibit an entirely different internal structure and transition behavior. Neutron reflectivity shows the absence of an ordered structure normal to the Si substrate in as-prepared PMEO₂MA-b-POEGMA₃₀₀ films. After exposure to D₂O vapor for 3 h and then increasing the temperature above its TT₁ and TT₂, the ordered structure is still not observed. Only a D₂O enrichment layer is formed close to the hydrophilic Si substrate. Such PMEO₂MA-b-POEGMA₃₀₀ films show a linear shrinkage between TT₁ and TT₂ in a D₂O vapor atmosphere. This special behavior can be attributed to the synergistic effect between the restrained collapse of the PMEO₂MA blocks by the still swollen POEGMA₃₀₀ blocks and the impedance of chain arrangement by the Si substrate. Based on this unique behavior, spin-coated PMEO₂MA-b-POEGMA₃₀₀ films are further prepared into a temperature sensor by implementing Ag electrodes. Its resistance decreases linearly with temperature between TT₁ and TT₂.

Stimuli-enabled switch-like paracetamol electrochemical sensor based on thermosensitive polymer and MWCNTs-GQDs composite nanomaterial

A temperature-controlled switchable electrochemical sensor was constructed based on a composite film consisting of thermosensitive block polymer poly(styrene-b-(N-isopropylacrylamide)-b-styrene) (PS-PNIPAm-PS), carboxylated multi-walled carbon nanotubes (MWCNTs-COOH) and amino-functionalized graphene quantum dots (N-GQDs). The prepared sensor showed good temperature sensitivity and reversibility in sensing paracetamol. In the low temperature environment, the polymer stretched to bury the electroactive sites of the carbon nanocomposite, and the paracetamol could not pass through the polymer to achieve electronic exchange, representing the "closed" state. Conversely, in the high temperature environment, the polymer shrank to expose the electroactive sites and enlarge background currents, the paracetamol was able to undergo the redox reaction normally and generate the response current, representing the "on" state. In addition, the sensor had a wide detection range (0.1 to 7.0 μM and 7.0 to 103.0 μM) and a low LOD of 66 nM for paracetamol. This switch-like sensor provided a novel idea for the application of thermosensitive polymers.

Triggering the Shrinking/Swelling Process in Thin Gel Layers on Conducting Surfaces by Applying an Appropriate Potential

Negatively charged, pH-sensitive, very thin gel layers with accumulated hexaammineruthenium (II)/(III) were deposited on conducting surfaces. The gel was synthesized by applying an electrochemically induced free-radical polymerization method. This method allowed covering the electrode surface with an uniform and compact layer. The modified electrodes exhibited excellent current switch on/off behavior in response to changes in pH. However, the main goal of this study was to achieve the control of the layer thickness by changing the oxidation state of hexaammineruthenium. The layers could be reversibly swollen/shrinked by applying appropriate potentials.

Electrochemical dopamine sensor based on the use of a thermosensitive polymer and an nanocomposite prepared from multiwalled carbon nanotubes and graphene oxide

An electrochemical dopamine sensor with a temperature-controlled switch was constructed by using a mixture of thermo-sensitive block copolymers (type tBA-PDEA-tBA), graphene oxide (GO) and multi-walled carbon nanotubes (MWCNTs). If the temperature is below 26 °C, the polymer on the glassy carbon electrode (GCE) is stretched, the distance between the MWCNTs is large, and the charge transfer resistance (R_ct) of the composite also is large. In the presence of dopamine, the electron transfer at the electrode is strongly retarded and in the "off" state. At above 38 °C, the polymer is shrunk and the R_ct is much smaller. The presence of dopamine results in a rapid electron transfer at the GCE, and this is referred to as the "on" state. At temperatures between 26 and 38 °C, the polymer shrinks slightly and has a "spring-like" state. There is a linear relationship between the response current (typically measured at a potential as low as 0.16 V vs. Ag/AgCl) and temperature. The response to dopamine is linear in the 0.06 to 4.2 μM and 4.2 to 18.2 μM concentration range, and the detection limit is 42 nM. Conceivably, this approach provides a novel approach towards the design of electrochemical sensors based on the use of thermo-sensitive polymers. Graphical abstract Schematic presentation of reversible and temperature-controlled electrochemical response of dopamine on the thermo-sensitive block copolymers (tBA-PDEA-tBA) / multi-walled carbon nanotubes (MWCNTs) / graphene oxide (GO) / glassy carbon electrode (GCE).

Design, synthesis and fluorescence behavior of novel chemosensor with a thieno[2,3-b]thiophene fluorophore

A novel chemosensor with a thieno[2,3-b] thiophene fluorophore bearing an oxazoline receptor exhibited good selectivity to the dichromate anion, and also shows enantioselectivity for mandelic acid with a chiral receptor.

Facile and large scale in situ synthesis of the thermal responsive fluorescent SiNPs/PNIPAM hydrogels

A new type of F-SiNPs/poly(N-isopropylacrylamide) (F-SiNPs/PNIPAM) hydrogel was prepared byin situpolymerization. The composite hydrogels display visible thermal-sensitive phase transition properties.