A Highly Sensitive Ascorbic Acid Sensor Based on Hierarchical Polyaniline Coated Halloysite Nanotubes Prepared by Electrophoretic Deposition

Halloysite-TiO2 Nanocomposites for Water Treatment: A Review

Halloysite nanotubes (HNTs) are clay minerals with a tubular structure that can be used for many different applications in place of carbon nanotubes. Indeed, HNTs display low/non-toxicity, are biocompatible, and can be easily prepared. Moreover, the aluminum and silica groups present on HNTs' inner and outer surfaces facilitate the interaction with various functional agents, such as alkalis, organosilanes, polymers, surfactants, and nanomaterials. This allows the deposition of different materials, for instance, metal and non-metal oxides, on different substrate types. This review article first briefly presents HNTs' general structure and the various applications described in the last 20 years (e.g., drug delivery, medical implants, and energy storage). Then, it discusses in detail HNT applications for water purification (inorganic and organic pollutants). It focuses particularly on HNT-TiO₂ composites that are considered very promising photocatalysts due to their high specific surface area and adsorption capacity, large pore volume, good stability, and mechanical features.

Transferring and Retaining of Different Polyaniline Nanofeatures via Electrophoretic Deposition for Enhanced Sensing Performance

Nanofeatured polyaniline (PANI) electrodes have demonstrated impressive sensing performance due to the enhanced electrolyte diffusion and ion transport. However, the retaining of these nanostructures on substrates via electrophoretic deposition (EPD) faces an insurmountable challenge from the involved dedoping process. Here, camphorsulfonic acid is utilized with high steric effects to dope PANI (PANI-CSA) that can be directly used EPD without involving a dedoping process. Five different nanofeatures (sea cucumber-like, nanofiber, amorphous, nanotube, and nanorod) are synthesized, and they have been all successfully transferred onto indium tin oxide substrate in a formic acid/acetonitrile system, namely a morphology memory effect. The mechanism of retaining these nanofeatures is revealed, which is realized via the processes of dissolution of PANI-CSA, codoping and solvation, and reassembly of basic units into the original nanofeature. The enhanced protonation level by the codoping of formic acid and solvation of acetonitrile plays the key role in retaining these nanofeatures. This method is also applicable to transfer PANI/gold nanorod composites (PANI-CSA/AuNRs). The PANI-CSA/AuNRs electrode as an ascorbic acid sensor has shown an excellent sensing performance with a sensitivity up to 872.7 µA mm^-1 cm^-2 and a detection limit of as low as 0.18 × 10^-6 m.

Functionally modified halloysite nanotubes for personalized bioapplications

Halloysite nanotubes (HNTs) are naturally aluminosilicate clay minerals that have the benefits of large surface areas, high mechanical properties, easy functionalization, and high biocompatibility, HNTs have been developed as multifunctional nanoplatforms for various bioapplications. Although some reviews have summarized the properties and bioapplications of HNTs, it remains unclear how to functionalize the modifications of HNTs for their personalized bioapplications. In this review, based on the physicochemical properties of HNTs, we summarized the methods of functionalized modifications (surface modification and structure modification) on HNTs. Also, we highlighted their personalized bioapplications (anti-bacterial, anti-inflammatory, wound healing, cancer theranostics, bone regenerative, and biosensing) by stressing on the main roles of HNTs. Finally, we provide perspectives on the future of functionalized modifications of HNTs for docking specific biological applications.

Effect of Ultrasonication on the Morphology, Mechanical Property, Ionic Conductivity, and Flame Retardancy of PEO-LiCF3SO3-Halloysite Nanotube Composites for Use as Solid Polymer Electrolyte

PEO-LiCF₃SO₃-halloysite nanotube (HNT) composites were fabricated by solution casting together with hot compression to form a solid polymer electrolyte (SPE) membrane. Different ultrasonic exposure times were used to disperse HNT nanoparticles in the PEO-20%LiCF₃SO₃-HNT composite solutions prior to casting. An exposure time of 15 min gave the highest ionic conductivity in the SPE membrane, the ionic conductivity significantly increased by two orders of magnitude from 6.6 × 10⁻⁶ to 1.1 × 10⁻⁴ S/cm. TEM, FE-SEM, and EDS-mapping were used to study the dispersion of HNTs in the SPE membrane. ATR-FTIR revealed that the bonding of PEO-LiCF₃SO₃ and PEO-HNT was created. XRD and DSC showed a reduction in the crystallinity of PEO due to HNT addition. The ultrasonication for an optimal period gave uniform dispersion of HNT, reduced the polymer crystallinity and strengthened the tensile property of SPE membrane. Moreover, the electrochemical stability, flame retardance and dimensional stability were improved by the addition of HNT and by ultrasonication.

Evaluation of membrane tailored with biocompatible halloysite‒polyaniline nanomaterial for efficient removal of carcinogenic disinfection by‒products precursor from water

Humic acid (HA) is the main component of natural organic matter that generates carcinogenic by‒products during disinfection and its removal from water resources is challenging. Biocompatible halloysite (HNTs) nanomaterial decorated with polyaniline (HNTs‒PANI) was synthesized via polymerization technique. HNTs‒PANI was added to prepare polyethersulfone mixed matrix membranes (MMMs). The influence of HNTs‒PANI concentration on HA removal efficiency was studied by varying the HNTs‒PANI (0.5, 1 and 1.5 wt%). The characterization studies of MMMs revealed that the addition of HNTs‒PANI improved the morphology of the membranes, surface properties, chemical stability and thermal property. The amine and hydroxyl groups within the MMMs improved the membrane wettability. The addition of HNTs‒PANI within the MMMs had significantly enhanced the pure water flux and HA filtration. YHP2 MMM with 1 wt% of HNTs‒PANI demonstrated sieving coefficient of 0.10 and the highest HA removal efficiency of 91% greater than the neat PES membrane. Furthermore, the antifouling property of the MMMs was studied using HA as foulant. 1 wt% of HNTs‒PANI added MMM showed a high flux recovery ratio (94.9%) with low total fouling of 12% and low irreversible fouling of 5%, respectively. Thus, HNTs‒PANI was an efficient nanomaterial for enhancing the pure water flux, removal efficiency and antifouling property to treat water contaminated with HA.

A portable ascorbic acid in sweat analysis system based on highly crystalline conductive nickel-based metal-organic framework (Ni-MOF)

Conductive metal-organic frameworks can provide unique porous structures, large pore volumes, many catalytically active sites and high crystallinity, and so are becoming increasingly important and attractive as electrocatalytic materials. The present work synthesized nanorods of the conductive compound Ni₃(2,3,6,7,10,11-hexaiminotriphenylene)₂ (Ni₃(HITP)₂) with a high degree of crystallinity from HITP ligands and Ni²⁺ ions. Screen-printed electrodes made with this material were employed to fabricate an enzyme-free sensor for the detection of ascorbic acid (AA). The sensor exhibited good catalytic activity during the electrocatalytic analysis of AA in alkaline media, attributed to the synergistic effect of highly active Ni-N₄ catalytic sites in the nanorods, the two-dimensional superimposed honeycomb lattice of the Ni₃(HITP)₂, and the large specific surface area of this material. The latter property facilitated efficient electron transfer during catalytic oxidation. A portable electrochemical AA detection system was developed using Ni₃(HITP)₂ as the electrode material together with application-specific integrated circuits and a smartphone application with App. Good sensing performance was obtained, including a wide linear range (2-200 μM) with high sensitivity (0.814 μA μM^-1 cm^-2), and low detection limit (1 μM). This system can be used to monitor AA levels and trends in sweat to assess vitamin C intake as a part of personal health management.

Wearable Self-Powered Smart Sensors for Portable Nutrition Monitoring

Self-powered sensors have attracted great attention in the field of analysis owing to the necessity of power resources for the routine use of sensor devices. However, it is still challenging to construct wearable self-powered sensors in a simple and efficient way. Herein, wearable self-powered textile smart sensors based on advanced bifunctional polyaniline/reduced graphene oxide (PANI/RGO) films have been successfully developed for remote real-time detection of vitamin C. Specifically, a pH-assisted oil/water (O/W) self-assembly strategy was proposed to boost the O/W self-assembled PANI/RGO films via proton regulation. The as-obtained PANI/RGO films could be directly loaded on the textile substrate, with good capacitive and biosensing performance due to the multifunctionality of PANI and RGO, respectively. Moreover, both wearable power supply devices and wearable biosensors based on PANI/RGO films possess good electrochemical performance, which paves the way for the actual application of self-powered nutrition monitoring. Significantly, obvious signals have been obtained in the detection of vitamin C beverages, exhibiting promising application values in daily nutrition track necessities. Prospectively, this study would provide an effective and simple strategy for integrating wearable self-powered sensors, and the developed smart sensing system is an ideal choice for the portable detection of nutrition.

In-situelectrochemical polymerization of aniline on flexible conductive substrates for supercapacitors and non-enzymatic ascorbic acid sensors

Polyaniline, as a kind of conductive polymer with commercial application prospects, is still under researches in its synthesis and applications. In this work, polyaniline was fabricated on flexible substrates including carbon cloths and polyethylene naphthalate byin situelectropolymerization method. The synthesized flexible electrodes were characterized by scanning electron microscopy, High resolution transmission electron microscope, atomic force microscope, Fourier transform infrared, x-ray diffraction, and x-ray photoelectron spectroscopy. Owing to the conductivity and the reversible redox property, the polyaniline/carbon cloth electrodes show excellent properties such as decent supercapacitor performance and good detection capability toward ascorbic acid. As supercapacitors, the electrodes exhibit a specific capacitance as high as 776 F g^-1at a current density of 1 A g^-1and a long cycle life of 20 000 times in the three-electrode system. As ascorbic acid sensors, the flexible electrodes demonstrate stable response to ascorbic acid in the range of 1-3000μM with an outstanding sensitivity (4228μA mM^-1cm^-2), low detection limit (1μM), and a fast response time. This work holds promise for high-performance and low-cost flexible electrodes for both supercapacitors and non-enzymatic ascorbic acid sensors, and may inspire inventions of self-powered electrochemical sensor.

Integrating Halloysite Nanostraws in Porous Catalyst Supports to Enhance Molecular Transport

In many porous catalyst supports, the accessibility of interior catalytic sites to reactant species could be restricted due to limitations of reactant transport through pores comparable to reactant dimensions. The interplay between reaction and diffusion in porous catalysts is defined through the Thiele modulus and the effectiveness factor, with diffusional restrictions leading to high Thiele moduli, reduced effectivess factors, and a reduction in the observed reaction rate. We demonstrate a method to integrate ceramic nanostraws into the interior of ordered mesoporous silica MCM-41 to mitigate diffusional restrictions. The nanostraws are the natural aluminosilicate tubular clay minerals known as halloysite. Such halloysite nanotubes (HNTs) have a lumen diameter of 15-30 nm, which is significantly larger than the 2-4 nm pores of MCM-41, thus facilitating entry and egress of larger molecules to the interior of the pellet. The method of integrating HNT nanostraws into MCM-41 is through a ship-in-a-bottle approach of synthesizing MCM-41 in the confined volume of an aerosol droplet that contains HNT nanotubes. The concept is applied to a system in which microcrystallites of Ni@ZSM-5 are incorporated into MCM-41. Using the liquid phase reduction of nitrophenol as a model reaction catalyzed by Ni@ZSM-5, we show that the insertion of HNT nanostraws into this composite leads to a 50% increase in the effectiveness factor. The process of integrating nanostraws into MCM-41 through the aerosol-assisted approach is a one-step facile method that complements traditional catalyst preparation techniques. The facile and scalable synthesis technique toward the mitigation of diffusional restrictions has implications to catalysis and separation technologies.

A nanocomposite of NiFe2O4-PANI as a duo active electrocatalyst toward the sensitive colorimetric and electrochemical sensing of ascorbic acid

A non-enzymatic ascorbic acid sensor using a nickel ferrite/PANI (NF-PANI) nanocomposite and based on colorimetric and electrochemical sensing methods was investigated in this study. The nanocomposite was prepared by an in situ polymerization and utilized as an electrocatalyst to sense ascorbic acid (AA) through the peroxidase mimic sensing of H₂O₂ in the presence of 3,5,3,5-tetramethylbenzidine (TMB) as a coloring agent. It was also utilized to detect AA present in real samples prepared from fruit extracts, commercial beverages, and vitamin-C tablets. The limit of detection (LoD) for AA sensing by the peroxidase mimic method was found to be 232 nM. The relative standard deviation (RSD) calculated for analysis of the real samples analysis ranged from 1.7-3.2%. Similarly, the electrochemical sensing of AA by NF-PANI was examined by cyclic voltammetric, chronoamperometric, and differential pulse voltammetric analyses. The LoD for the electrochemical method applied to AA sensing was 423 nM. The nanocomposite functioned as an effective electrocatalytic sensing agent in both methods to selectively detect AA due to the combined effect of NF and PANI. Thus, it was shown that the nanocomposites could be utilized for the laboratory-based detection of AA by various methods and could give rapid results.

Electrochemical (bio) sensors go green

Electrochemical (bio) sensors are now widely acknowledged as a sensitive detection tool for disease diagnosis as well as the detection of numerous species of pharmaceutical, clinical, industrial, food, and environmental origin. The term 'green' demonstrates the development of electrochemical (bio) sensing platforms utilizing biodegradable and sustainable materials. Development of green sensing platforms is one of the most active areas of research minimizing the use of toxic/hazardous reagents and solvent systems, thereby further reducing the production of chemical wastes in sensor fabrication. The present review includes green electrochemical (bio) sensors which are based on firstly, green sensors comprising natural and non-hazardous materials (e.g., paper/clay/zeolites/biowastes), secondly sensors based on nanomaterials synthesized by green methods and lastly sensors constituting green solvents (e.g., ionic liquids/deep eutectic solvents). Electrochemical performances of such green sensors and their benefits such as biodegradability, non-toxicity, sustainability, low-cost, sensitive surfaces, etc. Have been discussed for quantification of various target analytes. Associated challenges, possible solutions, and opportunities towards fabricating green electrochemical sensors and biosensors have been provided in the conclusion section.

Halloysite nanotubes as a nature's boon for biomedical applications

The arena of biomedical science has long been in quest of innovative mediums for diagnostic and therapeutic applications. The latest being the use of nanomaterials for such applications, thereby giving rise to the branch of nanomedicine. Halloysite nanotubes (HNTs) are naturally occurring tubular clay nanomaterials, made of aluminosilicate kaolin sheets rolled several times. The aluminol and siloxane groups on the surface of HNT facilitate the formation of hydrogen bonding with the biomaterials onto its surface. These properties render HNT pivotal in diverse range of applications, such as in environmental sciences, waste-water treatment, dye removal, nanoelectronics and fabrication of nanocomposites, catalytic studies, as glass coatings or anticorrosive coatings, in cosmetics, as flame retardants, stimuli response, and forensic sciences. The specific properties of HNT also lead to numerous applications in biomedicine and nanomedicine, namely drug delivery, gene delivery, tissue engineering, cancer and stem cells isolation, and bioimaging. In this review, recent developments in the use of HNT for various nanomedicinal applications have been discussed.

Surface modified halloysite nanotubes: A flexible interface for biological, environmental and catalytic applications

Halloysite Nanotubes (HNTs) are clay minerals that possess unique chemical composition and a tubular structure due to which, they have recently emerged as a potential nanomaterial for umpteen applications. Over the years, the myriad applications of HNT have been realized through the surface modification of HNT, which involves the modification of nanotube's inner lumen and the outer surface with different functional compounds. The presence of aluminum and silica groups on the inner and outer surface of HNT enhance the interfacial relationship of the nanotube with different functional agents. Compounds such as alkalis, organosilanes, polymers, compounds of biological origin, surfactants and nanomaterials have been used for the modification of the inner lumen and the outer surface of HNT. The strategies change the constitution of HNT's surface either through micro-disintegration of the surface or by introducing additional functional groups on the surface, which further enhances their potential to be used as a flexible interface for different applications. In this review, the different surface modification strategies of the outer surface and the inner lumen that have been employed over the years have been discussed. The biological, environmental and catalytic applications of these surface modified HNTs with such versatile interface in the past two years have been elaborately discussed as well.

Materials and Techniques for Implantable Nutrient Sensing Using Flexible Sensors Integrated with Metal-Organic Frameworks

The combination of novel materials with flexible electronic technology may yield new concepts of flexible electronic devices that effectively detect various biological chemicals to facilitate understanding of biological processes and conduct health monitoring. This paper demonstrates single- or multichannel implantable flexible sensors that are surface modified with conductive metal-organic frameworks (MOFs) such as copper-MOF and cobalt-MOF with large surface area, high porosity, and tunable catalysis capability. The sensors can monitor important nutriments such as ascorbicacid, glycine, l-tryptophan (l-Trp), and glucose with detection resolutions of 14.97, 0.71, 4.14, and 54.60 × 10^-6 m, respectively. In addition, they offer sensing capability even under extreme deformation and complex surrounding environment with continuous monitoring capability for 20 d due to minimized use of biological active chemicals. Experiments using live cells and animals indicate that the MOF-modified sensors are biologically safe to cells, and can detect l-Trp in blood and interstitial fluid. This work represents the first effort in integrating MOFs with flexible sensors to achieve highly specific and sensitive implantable electrochemical detection and may inspire appearance of more flexible electronic devices with enhanced capability in sensing, energy storage, and catalysis using various properties of MOFs.

Reduced graphene oxide-polyaniline film as enhanced sensing interface for the detection of loop-mediated-isothermal-amplification products by open circuit potential measurement

The development of low cost, portable diagnostic tools for in-field detection of viruses and other pathogenic microorganisms is in great demand but remains challenging. In this study, a novel approach based on reduced graphene oxide-polyaniline (rGO-PANi) film for the in situ detection of loop-mediated-isothermal-amplification (LAMP) products by means of open circuit potential measurement is proposed. The pH-sensitive conducting polymer PANi was electro-deposited onto rGO coated screen printed electrodes and tuned to be at the emeraldine state at which the pH sensitivity was maximized. By combining PANi and rGO, the pH sensitivity of the system was modulated up to about −64 mV per pH unit. This enabled the number of amplified amplicons resulting from the isothermal amplification process to be monitored. The sensor was then examined for monitoring LAMP reactions using Hepatitis B virus (HBV) as a model. This simple, low-cost, reproducible and sensitive interfacing layer is expected to provide a new possibility for designing point-of-care sensors under limited-resource conditions.

A novel disposable sensor based on reduced graphene oxide-polyaniline (rGO-PANi) for detection of loop-mediated-isothermal-amplification (LAMP) products.

Nanomaterial-doped conducting polymers for electrochemical sensors and biosensors

This review summarizes recent advances in the development of electrochemical sensors and biosensors based on nanomaterial doped conducting polymers.