Transparent Electrodes Printed with Nanocrystal Inks for Flexible Smart Devices

Transparent electrodes (TEs) are crucial in a wide range of modern electronic and optoelectronic devices. However, traditional TEs cannot meet the requirements of smart devices under development in unique fields, such as electronic skins, wearable electronics, robotic skins, flexible and stretchable displays, and solar cells. Emerging TEs printed with nanocrystal (NC) inks are inexpensive and compatible with solution processes, and have huge potential in flexible, stretchable, and wearable devices. Every development in ink‐based electrodes makes them more competitive for practical applications in various smart devices. Herein, we provide an overview of emergent ink‐based electrodes, such as transparent conducting oxides, metal nanowires, graphene, and carbon nanotubes, and their application in solution‐based flexible and stretchable devices.     

Fabrication of Miniaturised Screen‐printed Glucose Biosensors,Using a Water‐based Ink,and the Evaluation of their Electrochemical Behaviour

This paper describes a simple, convenient approach to the fabrication of microband electrodes and microband biosensors based on screen printing technology. These devices were printed in a three‐electrode configuration on one strip; a silver/silver chloride electrode and carbon counter electrode served as reference and counter electrodes respectively. The working electrodes were fabricated by screen‐printing a water‐based carbon ink containing cobalt phthalocyanine for hydrogen peroxide detection. These were converted into a glucose microband biosensor by the addition of glucose oxidase into the carbon ink. In this paper, we discuss the fabrication and application of glucose microband electrodes for the determination of glucose in cell media. The dimensions (100–400 microns) of the microband electrodes result in radial diffusion, which results in steady state responses in the absence of stirring. The microband biosensors were investigated in cell media containing different concentrations of glucose using chronoamperometry. The device shows linearity for glucose determination in the range 0.5 mM to 2.5 mM in cell media. The screen‐printed microband biosensor design holds promise as a generic platform for future applications in cell toxicity studies.     

Development of an on-column affinity smart polymer gel glucose sensor

An on-column affinity smart polymer gel glucose sensor was developed as a non-enzymatic glucose sensor. A copolymer of 3-acrylamidophenylboronic acid and acrylamide, the so called "smart polymer", was synthesized in situ in a 5 cm long capillary tube with a detection window to provide the on-column detection. The optical density of this semitransparent affinity smart polymer gel, coated inside the tube, decreased with increasing glucose concentration and was detected using a UV-vis detector at 500 nm. The capillary tube was incorporated into a flow injection system. Under optimum conditions, a linear dynamic range of 0.5-16.0mM with a limit of detection of 0.5mM (S/N ≥ 3) was obtained. A single coated affinity smart polymer gel had good stability for up to 250 consecutive injections with relative standard deviation of less than 5%. The analysis time for each injection was 6 min. Ten glucose samples prepared in distilled water were analyzed by the developed method and the results compared well with those obtained from the conventional dinitrosalicylic acid (DNS) method (P>0.05). Real urine samples with known glucose levels were analyzed and the developed sensor provided comparable results to those from the normal strip test technique. Acceptable percentage recoveries, ranging from 88 ± 2% to 103 ± 4% from the spiked urine sample, were obtained.     

Toward a glucose biosensor based on surface-enhanced Raman scattering

This work presents the first step toward a glucose biosensor using surface-enhanced Raman spectroscopy (SERS). Historically, glucose has been extremely difficult to detect by SERS because it has a small normal Raman cross section and adsorbs weakly or not at all to bare silver surfaces. In this paper, we report the first systematic study of the direct detection of glucose using SERS. Glucose is partitioned into an alkanethiol monolayer adsorbed on a silver film over nanosphere (AgFON) surface and thereby, it is preconcentrated within the 0-4 nm thick zone of electromagnetic field enhancement. The experiments presented herein utilize leave-one-out partial least-squares (LOO-PLS) analysis to demonstrate quantitative glucose detection both over a large (0-250 mM) and clinically relevant (0-25 mM) concentration range. The root-mean-squared error of prediction (RMSEP) of 1.8 mM (33.1 mg/dL) in the clinical study is near that desired for medical applications (1 mM, 18 mg/dL). Future studies will advance toward true in vivo, real time, minimally invasive sensing.     

A novel non-enzymatic glucose sensor modified with Fe2O3 nanowire arrays

Fe(2)O(3) was generally considered to be biologically and electrochemically inert, and its electrocatalytic functionality has been rarely realized directly in the past. In this work, Fe(2)O(3) nanowire arrays were synthesized and electrochemically characterized. The as prepared Fe(2)O(3) nanomaterial was proved to be an ideal electrode material due to the intrinsic peroxidase-like catalytic activity. The Fe(2)O(3) nanowire array modified glucose sensor exhibited excellent biocatalytic performance towards the oxidation of glucose with a response time of <6 s, a linear range between 0.015-8 mM, and sensitivity of 726.9 μA mM(-1)cm(-1). Additionally, a high sensing selectivity towards glucose oxidation in the presence of ascorbic acid (AA) and dopamine (DA) has also been obtained at their maximum physiological concentrations, which makes the Fe(2)O(3) nanomaterial promising for the development of effective electrochemical sensors for practical applications.     

Multilayer assembly of Prussian blue nanoclusters and enzyme-immobilized poly(toluidine blue) films and its application in glucose biosensor construction

A multilayered glucose biosensor via sequential deposition of Prussian blue (PB) nanoclusters and enzyme-immobilized poly(toluidine blue) films was constructed on a bare Au electrode using electrochemical methods. The whole configuration of the present biosensor can be considered as an integration of several independent hydrogen peroxide sensing elements. In each sensing element, the poly(toluidine blue) film functioned as both the supporting matrix for the glucose oxidase immobilization and the inhibitor for the diffusion of interferences, such as ascorbic acid and uric acid. Meanwhile, the deposited Prussian blue nanocluster layers acts as a catalyst for the electrochemical reduction of hydrogen peroxide formed from enzymatic reaction. Performance of the whole multilayer configuration can be tailored by artificially arranging the sensing elements assembled on the electrode. Under optimal conditions, the biosensors exhibit a linear relationship in the range of 1 x 10(-4) to 1 x 10(-2) mol/L with the detection limit down to 10(-5) mol/L. A rapid response for glucose could be achieved in less than 3 s. For 1 mM glucose, 0.5 mM acetaminophen, 0.2 mM uric acid, and 0.1 mM ascorbic acid have no obvious interferences (<5%) for glucose detection at an optimized detection potential. The present multilayered glucose biosensor with a high selectivity and sensitivity is promising for practical applications.     

Soft contact lens biosensor for in situ monitoring of tear glucose as non-invasive blood sugar assessment

A contact lens (CL) biosensor for in situ monitoring of tear glucose was fabricated and tested. Biocompatible 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer and polydimethyl siloxane (PDMS) were employed as the biosensor material. The biosensor consists of a flexible Pt working electrode and a Ag/AgCl reference/counter electrode, which were formed by micro-electro-mechanical systems (MEMS) technique. The electrode at the sensing region was modified with glucose oxidase (GOD). The CL biosensor showed a good relationship between the output current and glucose concentration in a range of 0.03-5.0 mM, with a correlation coefficient of 0.999. The calibration range covered the reported tear glucose concentrations in normal and diabetic patients. Also, the CL biosensor was applied to a rabbit for the purpose of tear glucose monitoring. The basal tear glucose was estimated to 0.11 mM. Also, the change of tear glucose induced by the change of blood sugar level was assessed by the oral glucose tolerance test. As a result, tear glucose level increased with a delay of 10 min from blood sugar level. The result showed that the CL biosensor is expected to provide further detailed information about the relationship between dynamics of blood glucose and tear glucose.     

Fluorescence sensors for monosaccharides based on the 6-methylquinolinium nucleus and boronic acid moiety: potential application to ophthalmic diagnostics

Continuous monitoring of glucose levels in human physiology is important for the long-term management of diabetes. New signaling methods/probes may provide an improved technology to monitor glucose and other physiologically important analytes. The glucose sensing probes, BMQBAs, fabricated using the 6-methylquinolinium moiety as a fluorescent indicator, and boronic acid as a chelating group, may have versatile applications in glucose sensing because of their unique properties. In this paper we discuss the design logic, synthesis, characterization and spectral properties of three new isomeric glucose sensors (BMQBAs), and a control compound (BMQ) in the presence and absence of sugars. The sensing ability of the new probes is based on a charge neutralization and stabilization mechanism upon sugar binding. The new probes have attractive fluorescence quantum yields, are highly water-soluble, and have spectral characteristics compatible with cheap and portable LEDs and LDs. One of the probes, o-BMQBA, has a sugar bound pK_a of 6.1, and a dissociation constant K_D of 100 mM glucose. These probes have been designed specifically to respond to tear glucose in a contact lens polymer for ophthalmic glucose monitoring, where the reduced sugar bound pK_a affords for sensing, in a lens environment that we have previously shown to be mildly acidic.     

Amperometric biosensor based on thermally activated polymer-stabilized metal nanoparticles

Polymer-stabilized Pd nanoparticles on carbon support were synthesized by a low thermal procedure that does not involve the utilization of a reducing agent such as NaBH₄ or hydrogen gas for the formation of the metallic nanoparticles. The Pd-catalyzed graphite particles were then mixed with known amounts of glucose oxidase (GOx) enzyme and Nafion to prepare a GOx-immobilized ink. A glassy carbon electrode (GCE) modified with the GOx ink was used to evaluate the performance of the biosensor electrode. The results of TEM and AFM show that the Pd nanoparticles are uniformly distributed on top of the substrate. Results are presented for sensing glucose through the voltammetric measurement of H₂O₂. Coupled with the simplicity of preparation, the biosensor exhibited high sensitivity and extended linear range for glucose measurement. Further, the electrochemical characteristics of the nanocomposite biosensor were evaluated with respect to the electrochemistry of potassium ferricyanide by cyclic voltammetry. Whereas the presence of polymer and Nafion improved the stability of both the ink and biosensor electrode, the concentration of glucose was measured without interferences from oxygen, ascorbic acid and uric acid because of the Nafion.     

Cast thin film biosensor design based on a Nafion backbone, a multiwalled carbon nanotube conduit, and a glucose oxidase function

Novel electroanalytical sensing nanobiocomposite materials are reported. These materials are prepared by mixing multiwalled carbon nanotubes (MWNTs), a Nafion cation exchanger, and glucose oxidase (GOD) in appropriate amounts. The MWNTs are cylindrical with a diameter in the range 40-60 nm and with a length of up to several micrometers, and they provide electrical conductivity. Nafion acts as a polymer backbone to give stable and homogeneous cast thin films. Both MWNTs and Nafion provide negative functionalities to bind to positively charged redox enzymes such as glucose oxidase. The resulting biosensing composite material is inexpensive, reliable, and easy to use. The homogeneity of the MWNT-Nafion-GOD nanobiocomposite films was characterized by atomic force microscopy (AFM). Amperometric transducers fabricated with these materials were characterized electrochemically using cyclic voltammetry and amperometry in the presence of hydrogen peroxide and in the presence of glucose. Their linear response to hydrogen peroxide was demonstrated. The glucose biosensor sensitivity was strongly influenced by the glucose oxidase concentration within the nanobiocomposite film. The optimized glucose biosensor (2.5 mg/mL GOD) displayed a sensitivity of 330 nA/mM, a linear range of up to 2 mM, a detection limit of 4 microM, and a response time of <3 s.     

Recent advances of multidimensional sensing: from design to applications

Recently, multidimensional(or multi-channel) sensing methodology has attracted broad attention in the field of analytical chemistry due to its fascinating merits. A variety of multidimensional sensors based on sensor arrays, lab-on-a-molecule/nanoparticle and smart chip strategies have been designed to differentiate chemical structure and property similar analytes and complex samples. Pattern recognition algorithms are usually used and allow these sensors to fulfill such proposes. In this review,the recent advances of multidimensional sensor devices were firstly summarized, and particularly focused on their design strategies and applications in monitoring of biological active molecules, biomarkers, microbes, foods and beverages, etc. Then,some limitations and possible solutions of multidimensional sensors were discussed. And finally, potential applications of this technique in the future were proposed. This review would help the readers who are interested in multidimensional sensing methodology to understand the research progresses and trends.     

Copper‐based Metal‐organic Framework for Non‐enzymatic Electrochemical Detection of Glucose

A non‐enzymatic electrochemical glucose sensor based on a Cu‐based metal‐organic framework (Cu‐MOF) modified electrode was developed. The Cu‐MOF was prepared by a simple ionothermal synthesis, and the characterizations of the Cu‐MOF were studied by Fourier transform infrared spectroscopy (FT‐IR), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), single‐crystal X‐ray powder diffraction (SCXRD), and X‐ray powder diffraction (XRD). Electrochemical behaviors of the Cu‐MOF modified electrode to glucose were measured by differential pulse voltammetry (DPV). The electrochemical results showed that the Cu‐MOF modified electrode exhibited an excellent electro‐catalytic oxidation towards glucose in the range of 0.06 μM to 5 mM with a sensitivity of 89 μA/mM cm² and a detection limit of 10.5 nM. Moreover, the fabricated sensor showed a high selectivity to the oxidation of glucose in coexistence with other interferences. The sensor was satisfactorily applied to the determination of glucose in urine samples. With the significant electrochemical performances, MOFs may provide a suitable platform in the construction of kinds of electrochemical sensors and/or biosensors and hold a great promise for sensing applications.     

Progress in patterned paper sizing for fabrication of paper-based microfluidic sensors

In this paper, we report the progress in using paper sizing chemistry to fabricate patterned paper for chemical and biological sensing applications. Patterned paper sizing uses paper sizing agents to selectively hydrophobize certain area of a sheet. The hydrophilic-hydrophobic contrast of the pattern so created has an excellent ability to control capillary penetration of aqueous liquids in channels of the pattern. Incorporating this idea with digital ink jet printing technique, a new fabrication method of paper-based microfluidic devices is established. Ink jet printing can deliver biomolecules and chemicals with precision into the microfluidic patterns to form biological/chemical sensing sites within the patterns, forming the complete sensing devices. This study shows the potential of combining paper sizing chemistry and ink jet printing to produce paper-based sensors at low cost and at commercial volume.     

Carbon nanomaterials for salivary-based biosensors: a review

In recent years, saliva has been introduced as an alternate to conventional biofluid assays owing to both accessibility and reliability with regard to the assessment of different biomarkers. The capability of immediate online collection and analysis of salivary biomarkers offers myriad benefits for clinical applications, resulting in the demands for quantifying salivary biomarkers rapidly and reliably with the help of biosensing technology. Carbon–nanomaterial based biosensors provide potential instruments for a non-aggressive pain-free style of saliva-dependent recognition to diagnoses, monitor, and formulate a therapeutic modality and for managing patients. This review covers the importance of carbon nanomaterial in fabricating salivary-based detectors applied clinically for diagnostics and therapeutics. The utilization of carbon nanomaterials comprising carbon dots (CDs), carbon nanotubes (CNTs), graphene, graphene oxide (GO), reduced graphene oxide (rGO) and graphitic carbon nitride (g-C₃N₄) in salivary-based detection has been highlighted here with up-to-date instances. These sensing systems are capable of detecting a vast range of molecules with clinical relevance, including glucose, hormones, amino acids, viruses, bacteria, cancer antigens, cancer biomarkers, dopamine, sialic acid, uric acid, etc. which were discussed in this paper.     

Fabrication of a non-enzymatic glucose sensor field-effect transistor based on vertically-oriented ZnO nanorods modified with Fe2O3

Herein, we report a non-enzymatic glucose sensor field-effect transistor (FET) based on vertically-oriented zinc oxide nanorods modified with iron oxide (Fe₂O₃-ZNRs). Compared with ZnO-based non-enzymatic glucose sensors, which show poor sensing performances, modification of ZnO with Fe₂O₃ dramatically enhances the sensing behavior of the fabricated non-enzymatic FET glucose sensor due to the excellent electrocatalytic nature of Fe₂O₃. The fabricated non-enzymatic FET sensor showed excellent catalytic activity for glucose detection under optimized conditions with a linear range up to 18 mM, detection limits down to ~ 12 μM, excellent selectivity, good reproducibility and long-term stability. Moreover, the fabricated FET sensor detected glucose in freshly drawn mouse whole blood and serum samples. The developed FET sensor has practical applications in real samples and the solution-based synthesis process is cost effective.     

A highly sensitive water-soluble system to sense glucose in aqueous solution

A glucose sensing switch is formed by water soluble conjugated polymer (PP-S-BINOL) and boronic acid-functionalized benzyl viologen (o-BBV). The two-component system shows a high sensitivity for glucose sensing with a 17-fold increase in the fluorescence intensity in the presence of 100 mM glucose.     

Electrochemical oxidative determination of 4-nitrophenol based on a glassy carbon electrode modified with a hydroxyapatite nanopowder

A non-enzymatic amperometric glucose is reported that is based on an glassy carbon electrode modified with a Cu-CuO nanowire (NW) composite. The morphology and the composition of the nanowire were characterized by scanning electron microscopy and X-ray diffraction, respectively. The modified electrode efficiently catalyzes the oxidation of glucose at less-positive potential (0.30 V) in 0.10 M NaOH solution in the absence of any enzymes or redox mediators. The sensor was successfully used for the amperometric sensing of glucose. Linear response was obtained over the concentration range from 0.1 to 12 mM. The common interfering agents ascorbic acid and uric acid do not interfere with the determination of glucose. The modified electrode features high sensitivity, low working potential, excellent stability, and fast amperometric sensing of glucose. Thus it is promising for the future development of non-enzymatic glucose sensors.     

MnO<Subscript>2</Subscript> Nanoparticles and Carbon Nanofibers Nanocomposites with High Sensing Performance Toward Glucose

By combining the advantages of manganese dioxide nanoparticles (MnO₂ NPs) and carbon nanofibers (CNFs), a biosensing electrode surface as a high-performance enzyme biosensor is designed in this work. MnO₂ NPs and CNFs nanocomposites (MnO₂–CNFs) were prepared by using a simple hydrothermal method and then were characterized by scanning electron microscopy, powder X-ray diffraction, fourier transform infrared spectroscopy, energy dispersive spectrometry and electrochemisty. The results showed that MnO₂ NPs are uniformly attached to the surface of CNFs. Meanwhile, the MnO₂–CNFs nanocomposites as a supporting matrix can provide an efficient and advantageous platform for electrochemical sensing applications. On the basis of the improved sensitivity of MnO₂–CNFs modified electrode toward H₂O₂ at low overpotential, a MnO₂–CNFs based glucose biosensor was fabricated by monitoring H₂O₂ produced by an enzymatic reaction between glucose oxidase and glucose. The constructed biosensor exhibited a linear calibration graph for glucose in a concentration range of 0.08–4.6 mM and a low detection limit of 0.015 mM. In addition, the biosensor showed other excellent characteristics, such as high sensitivity and selectivity, short response time, and the relative low apparent Michaelis–Menten constant. Analysis of human urine spiked with glucose at different concentration levels yielded recoveries between 101.0 and 104.8%.     

Nickel/Copper Bilayer‐modified Screen Printed Electrode for Glucose Determination in Flow Injection Analysis

This work reports about the performance of a Ni/Cu‐modified screen printed electrodes (SPE/Ni/Cu), prepared by physical vapor deposition (PVD) in an oblique angle configuration (OAD), for non‐enzymatic glucose sensing applications. SPE/Ni/Cu electrodes showed an excellent reversibility and a catalytic behavior for detection of glucose that were controlled by the diffusion of reactants up to the active sites at the electrode surface. The study with a flow injection analysis (FIA) setup of the main experimental variables affecting the detection process has shown that the developed electrode system had an excellent glucose sensitivity of 1.04 A M⁻¹cm⁻² (R²:0.999), a linear response up to 1 mM, a limit of detection of 0.33 μM and a time of analysis of ca. 30 s per sample. The selectivity of the sensor was checked against various interferences, including ascorbic acid, uric acid, acetaminophen and other sugars, in all cases with excellent results. The feasibility of using this sensor for practical applications was successfully confirmed by determining the glucose concentration in different commercial beverages.     

Novel chemico-mechanical approach towards long-term implantable glucose sensing

The proof of concept of a continuously sensing affinity device based on the glucose-dependent viscosity of a sensitive solution containing dextran and Concanavalin A has been successfully demonstrated. The biosensor incorporates a piezoelectric diaphragm and a flow-resisting microchannel for viscosity detection, and a free-standing Anodic Alumina Oxide (AAO) porous nano-membrane as glucose selective interface. Extensive in vitro glucose measurements between two physiologically relevant glucose concentrations, 2 mM and 9 mM (respectively hypo- and hyperglycemia), were successfully performed during 4 days. To the best of our knowledge, such reversibility and stability of glucose measurement over time had not been reported yet.