Application of surfactants in the electrochemical sensing and biosensing of biomolecules and drug molecules.

Realizing sensitive and efficient detection of biomolecules and drug molecules is of great significance. Among the detection methods that have been proposed, electrochemical sensing is favored for its outstanding advantages such as simple operation, low cost, fast response and high sensitivity. The unique structure and properties of surfactants have led to a wide range of applications in the field of electrochemical sensors and biosensors for biomolecules and drug molecules. Through the comparative analysis of reported works, this paper summarizes the application modes of surfactants in electrochemical sensors and biosensors for biomolecules and drug molecules, explores the possible electrocatalytic mechanism of their action, and looks forward to the development trend of their applications. This review is expected to provide some new ideas for subsequent related research work.

Fully Converting Graphite into Graphene Oxide Hydrogels by Preoxidation with Impure Manganese Dioxide.

Potassium permanganate (KMnO4) has been proved to be an efficient oxidant for converting graphite into graphite oxide, but its slow diffusion in the interlayer of graphite seriously restricts the production of graphene oxide (GO). Here, we demonstrate that the preoxidation of graphite by impure manganese dioxide (MnO2) in a mixture of concentrated sulfuric acid (H2SO4) and phosphorus pentoxide (P2O5) can efficiently improve the synthesis of GO when KMnO4 is employed as the oxidant. The prepared honey-like GO hydrogels possess a high yield of single-layer sheets, large sizes (average lateral size up to 20 µm), wide ranges of stable dispersion concentrations (from dilute solutions, viscous hydrogels, to dry films), and good conductivity after reduction (~2.9 × 10(4) S/m). The mechanism for the improved synthesis of GO by impure MnO2 was explored. The enhanced exfoliation and oxidation of graphite by oxidative Mn ions (mainly Mn(3+)), which are synergistically produced by the reaction of impure MnO2 with H2SO4 and P2O5, are found to be responsible for the improved synthesis of such GO hydrogels. Particularly, preoxidized graphite (POG) can be partially dispersed in water with sonication, which allows the facile construction of flexible and highly conductive graphene nanosheet film electrodes with excellent electrochemical sensing properties.

Ultrasensitive photoelectrochemical biosensing of multiple biomarkers on a single electrode by a light addressing strategy.

Ultrasensitive multiplexed detection of biomarkers on a single electrode is usually a great challenge for electrochemical sensors. Here, a light addressable photoelectrochemical sensor (LAPECS) for the sensitive detection of multiple DNA biomarkers on a single electrode was reported. The sensor was constructed through four steps: (1) immobilization of capture DNA (C-DNA) of different targets on different areas of a single large-sized gold film electrode, (2) recognition of each target DNA (T-DNA) and the corresponding biotin-labeled probe DNA (P-DNA) through hybridization, (3) reaction of the biotin-labeled probe DNA with a streptavidin-labeled all-carbon PEC bioprobe, and (4) PEC detection of multiple DNA targets one by one via a light addressing strategy. Through this principle, the LAPECS can achieve ultrasensitive detection of three DNA sequences related to hepatitis B (HBV), hepatitis C (HCV) and human immunodeficiency (HIV) viruses with a similar wide calibration range of 1.0 pM ∼ 0.01 µM and a low detection limit of 0.7 pM by using one kind of PEC bioprobe. Moreover, the detection throughput of LAPECS may be conveniently expanded by simply enlarging the size of the substrate electrode or reducing the size of the sensing arrays and the light beam. The present work thus demonstrates the promising applications of LAPECS in developing portable, sensitive, high-throughput, and cost-effective biosensing systems.

Rapid in situ detection of ultratrace 2,4-dinitrotoluene solids by a sandwiched paper-like electrochemical sensor.

This work reported the rapid in situ detection of ultratrace 2,4-dinitrotoluene (DNT) solids on various substrates by a sandwiched paper-like electrochemical sensor. The sensor, prepared by a simple electroless deposition method without using special instruments, possessed a unique thin-film structure of an insulated polyvinylidene fluoride (PVDF) membrane in between two gold (Au) conducting layers. The resulting gold-PVDF sandwich (GPVDFS) array exhibited excellent flexibility, porosity and electrochemical performance as a highly integrated dual-electrode sensor platform. The infiltration of nonvolatile ionic liquid (IL) electrolytes containing ferrocene (Fc) into the GPVDFS array produced a paper-like electrochemical sensor, which can directly detect ultratrace DNT solids on various substrate surfaces (e.g., plant leaves, gloves and metal knives) with detection limit as low as 0.33 ng/mm(2). The critical role of Fc in the detection of DNT at this dual-electrode sensor was explored. The compensating electrochemical oxidation of Fc at the counter/reference electrode was found to be essential to the reduction of DNT at the working electrode when IL electrolytes were employed. The present work thus demonstrated the promising applications of paper-based porous electrode arrays in developing IL-based electrochemical sensors for the in situ detection of analyte solids in complicated environments.

Preparation and application of a novel electrochemical sensing material based on surface chemistry of polyhydroquinone.

A new analogue of polydopamine (PDA), i.e., polyhydroquinone (PH2Q), was polymerized and its surface chemistry was studied by different ways of characterization. PH2Q was produced by the self-polymerization of H2Q mediated by dissolved oxygen, and the self-polymerization process was strongly dependent on the type and the pH value of the buffer solutions. PH2Q can not only achieve surface hydrophilization of different substrates like polyethylene terephthalate (PET) film, graphite strip, C12SH/Au and wax slice, but also possess several unique properties like reversible adsorption, good solubility and low cost. These properties made PH2Q an ideal polymeric modifier for the noncovalent functionalization of some nanomaterials. By simply grinding with PH2Q, pristine multi-walled carbon nanotubes (MWNTs) can be readily dispersed in water with high solubility and good stability. The resulting MWNT-PH2Q composite exhibited excellent electrochemical performance, which was employed for the simultaneous determination of dopamine (DA) and uric acid (UA).

Ultrasensitive all-carbon photoelectrochemical bioprobes for zeptomole immunosensing of tumor markers by an inexpensive visible laser light.

A novel enzyme-free and all-carbon photoelectrochemical (PEC) bioprobe, based on carboxylated multiwalled carbon nanotube-Congo red-fullerene nanohybrids (MWNTCOOH-CR-C60), for the ultrasensitive immunosensing of carcinoembryonic antigen (CEA) was reported. The MWNTCOOH-CR-C60 nanohybrids, prepared by mechanically grinding a mixture of MWNTCOOH, C60, and CR at a certain mass ratio, had good water dispersibility and high PEC conversion efficiency in visible light ranges. Covalent binding of the detection antibody of CEA on the MWNTCOOH-CR-C60 nanohybrids produced a sensitive PEC bioprobe for detection of CEA by sandwich immunosensing. The corresponding immunosensor, employing an inexpensive and portable green laser light, possessed a wide calibration range of 1.0 pg/mL~100.0 ng/mL and a low detection limit of 0.1 pg/mL (calculated 5 zmol for a 10.0 µL sample solution) (S/N = 3), which was successfully applied to the detection of CEA in serum samples from both healthy people and cancer patients. The present work thus demonstrated the promising application of fullerene-based nanocomposites in developing highly sensitive, environmentally friendly, and cost-effective PEC biosensors.

Inkjet printing of nanoporous gold electrode arrays on cellulose membranes for high-sensitive paper-like electrochemical oxygen sensors using ionic liquid electrolytes.

A simple approach to the mass production of nanoporous gold electrode arrays on cellulose membranes for electrochemical sensing of oxygen using ionic liquid (IL) electrolytes was established. The approach, combining the inkjet printing of gold nanoparticle (GNP) patterns with the self-catalytic growth of these patterns into conducting layers, can fabricate hundreds of self-designed gold arrays on cellulose membranes within several hours using an inexpensive inkjet printer. The resulting paper-based gold electrode arrays (PGEAs) had several unique properties as thin-film sensor platforms, including good conductivity, excellent flexibility, high integration, and low cost. The porous nature of PGEAs also allowed the addition of electrolytes from the back cellulose membrane side and controllably produced large three-phase electrolyte/electrode/gas interfaces at the front electrode side. A novel paper-based solid-state electrochemical oxygen (O(2)) sensor was therefore developed using an IL electrolyte, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF(6)). The sensor looked like a piece of paper but possessed high sensitivity for O(2) in a linear range from 0.054 to 0.177 v/v %, along with a low detection limit of 0.0075% and a short response time of less than 10 s, foreseeing its promising applications in developing cost-effective and environment-friendly paper-based electrochemical gas sensors.

A disposable electrochemical sensor for the determination of indole-3-acetic acid based on poly(safranine T)-reduced graphene oxide nanocomposite.

A disposable electrochemical sensor for the determination of indole-3-acetic acid (IAA) based on nanocomposites of reduced graphene oxide (rGO) and poly(safranine T) (PST) was reported. The sensor was prepared by coating a rGO film on a pre-anodized graphite electrode (AGE) through dipping-drying and electrodepositing a uniform PST layer on the rGO film. Scanning electron microscopic (SEM) and infrared spectroscopic (IR) characterizations indicated that PST-rGO formed a rough and crumpled composite film on AGE, which exhibited high sensitive response for the oxidation of IAA with 147-fold enhancement of the current signal compared with bare AGE. The voltammetric current has a good linear relationship with IAA concentration in the range 1.0×10(-7)-7.0×10(-6)M, with a low detection limit of 5.0×10(-8)M. This sensor has been applied to the determination of IAA in the extract samples of several plant leaves and the recoveries varied in the range of 97.71-103.43%.

Fabrication and application of poly(alizarin red S)-carbon nanotubes composite film based nitrite sensor.

In this work, a novel electrochemical nitrite sensor for sensitive determination of nitrite based on poly(alizarin red S)-multi-wall carbon nanotubes (PARS-MWNTs) composite film on the glassy carbon electrode was described. The surface morphologies of different electrodes were characterized by scanning electron microscopy. Cyclic voltammetry, chronocoulometry and linear sweep voltammetry were used to investigate the electrochemical response and oxidation mechanism of nitrite at the PARS-MWNTs composite film based sensor. The experimental parameters were optimized, such as electropolymerization pH value, film thickness and detection pH value et al. Under optimal working conditions, the oxidation peak current of nitrite linearly increased with its concentration in the range of 30 nM to 1.1 mM with a low detection limit of 2 nM. The PARS-MWNTs composite film based nitrite sensor was applied to the determination of nitrite in sausage, and the good recovery indicated that it may have practical applications in nitrite monitoring system.

Electrochemical behavior of biocatalytical composite based on heme-proteins, didodecyldimethylammonium bromide and room-temperature ionic liquid.

A novel biocompatible composite film based on a water-insoluble surfactant, didodecyldimethylammonium bromide (DDAB), and a hydrophobic room-temperature ionic liquid (RTIL), 1-hexyl-3-methyl-imidazolium hexafluorophosphate (HIMIMPF(6)), for the immobilization of biocatalytical proteins was reported. Differential scanning calorimetry (DSC) showed that the DDAB-HIMIMPF(6) composite film has higher thermal stability than the DDAB film alone. SEM images indicated that different microstructures existed between the DDAB film and the composite film, indicating the interaction between DDAB and RTILs. This composite can be used as the immobilization matrix of proteins and other biomacromolecules. Heme-proteins, including hemoglobin (Hb), myoglobin (Mb) and horseradish peroxidase (HRP), were used as model proteins for studying the electrochemical behaviors of the resulting biocatalytical composite films. In the case of Hb, a pair of well-defined quasi-reversible redox peaks was obtained when the composite film containing Hb was modified on a glassy carbon electrode. The formal potential (E degrees '), the surface coverage (Gamma(*)) and the electron transfer rate constant (k(s)) were calculated as -0.308V, 1.32x10(-11)molcm(-2) and 11.642s(-1), respectively. While, these parameters for Hb on DDAB films alone were -0.309V, 7.20x10(-12)molcm(-2) and 2.748s(-1), respectively. Therefore, the composite are more suitable for the direct electron transfer between Hb than DDAB alone. The native conformation and bioactivity of Hb adsorbed on the composite film was proved to be maintained, reflected by the unchanged ultraviolet-visible (UV-vis) as well as the catalytic activity toward hydrogen peroxide (H(2)O(2)) and nitric oxide (NO) compared with the free Hb molecules. Furthermore, Hb on the composite film are more sensitive for the detection of hydrogen peroxide (H(2)O(2)) and nitric oxide (NO) than that on DDAB film alone. The linear range of H(2)O(2) on Hb/DDAB-RTILs/GC electrode is from 0.5 to 57.5microM with linear regression equations I(microA)=0.149+0.00904C(microM), while, the linear range of H(2)O(2) on Hb/DDAB/GC electrode is from 0.5 to 57.5microM with linear regression equations I(microA)=0.0938+0.00553C(microM). For NO, its linear range on Hb/DDAB-RTILs/GC electrode is from 1.8 to 21.6microM with linear regression equations I(microA)=0.0937+0.0232C(microM). But its linear range on Hb/DDAB/GC electrode is from 1.8 to 21.6microM with linear regression equations I(microA)=0.0285+0.0167C(microM). Similar results were observed for Mb and HRP in the DDAB-HIMIMPF(6) composite film.

Direct electrochemical determination of methyl jasmonate in wheat spikelet at a nano-montmorillonite film modified electrode by derivative square wave voltammetry.

The direct electrochemical determination of methyl jasmonate (MeJA) at a nano-montmorillonite modified glassy carbon electrode (nano-MMT/GCE) is reported. The modified electrode, prepared by a simple casting-drying method and characterized by scanning electron microscope (SEM) and electrochemical impedance spectra (EIS), was proved to process a uniform nanostructured surface with a large surface area and a fast electron transfer rate. This electrode exhibited a sensitive electrochemical response for the direct oxidation of MeJA in 0.1 mol L(-1) HClO4, which could be further improved by using a derivative square wave voltammetry technique. Thus, a simple and fast electrochemical method for the determination of MeJA is proposed. Under optimal working conditions, the oxidation current of MeJA linearly increased with its concentration in the range of 7.0 × 10(-7)-1.0 × 10(-3) mol L(-1) with a detection limit of 5.0 × 10(-7) mol L(-1). This method had been applied to the determination of MeJA content in wheat spikelet samples.

Single-chain surfactant monolayer on carbon paste electrode and its application for the studies on the direct electron transfer of hemoglobin.

A variety of single-chain surfactants with different charge properties and tail lengths can spontaneously adsorb on the hydrophobic surface of carbon paste electrode and form stable monolayers on the electrode surface. Hemoglobin (Hb) was successfully immobilized on these surfactant monolayers to form stable protein-surfactant composite films regardless of the charge and the tail length of surfactants. The resulting surface-confined Hb exhibited well-defined direct electron-transfer behaviors in all positively, neutrally and negatively charged surfactant films, suggesting the important role of hydrophobic interactions in the adsorption of Hb on surfactant films. When the density of surfactant monolayers was controlled to be the same, Hb was found to possess a better direct electron-transfer behavior on monolayers of cationic surfactants with a longer tail length. This, in combination with the tunneling effect in the direct electron transfer of Hb on surfactant films, demonstrated that the adsorption of Hb on surfactant monolayers may be mainly achieved by the partial intercalation of Hb in the loose structures of surfactant films through hydrophobic interactions between the alkane chains of surfactants and the hydrophobic regions of Hb. The native conformation of Hb adsorbed on these surfactant films was proved to be unchanged, reflected by the unaltered ultraviolet-visible (UV-vis) and reflection-absorption infrared (RAIR) spectra, and by the catalytic activity toward hydrogen peroxide (H(2)O(2)) and nitric oxide (NO) in comparison with the free Hb molecules.

Nitric oxide sensor based on poly (p-phenylenevinylene) derivative modified electrode and its application in rat heart.

A novel nitric oxide (NO) electrochemical sensor was fabricated by electropolymerization with two-pulse potential method in aqueous solution containing poly (p-phenylenevinylene) derivative. The experimental results showed this sensor exhibits apparent current response to NO by cyclic voltammetry. Two anodic peaks potential are significantly shifted negatively compared with bare glass carbon electrode (GCE). Chronocoulometry were used to further validate the electron transfer process of NO oxidization on this sensor. The results indicated that oxidation of NO proceeds in two steps. The first step leading to NO(2)(-) involves one-electron transfer process and the second step corresponding to the oxidation of NO(2)(-) to NO(3)(-) involves two-electron transfer process. The amperometric response is linear with NO concentration ranging from 1.8x10(-7) to 1.0x10(-4) mol L(-1) with a linear coefficient of 0.9993. The detection limit of NO is 2.3x10(-8) mol L(-1) (S/N=3). After possible interferences were tested, NO release from the rat heart cells stimulated by L-arginine was measured using this sensor. The result is satisfactory.

Surface design of carbon nanotubes for optimizing the adsorption and electrochemical response of analytes.

Carbon nanotubes (CNTs) from different sources were dissolved in water with high solubility by Congo red (CR) via strong noncovalent pi-stacking interactions. The resulting CNTs were capable of forming uniform, compact, stable films on various substrates. This provided a chance to explore the relationship between the surface property of CNTs and the adsorptive behavior of analytes on CNTs without considering the influence of film structures or free additives. Electrochemical behaviors of several small biomolecules and glucose oxidase (GOD) on various CR-functionalized CNT films were examined. The results showed that both the hydrophobic structural defect sites and the hydrophilic oxygen-containing groups were the electroactive sites of CNTs, which was further proven by UV-vis and FTIR spectra. Moreover, the surface properties of CNTs could be conveniently designed by simple pretreatments for optimizing the adsorption and the electrochemical response of analytes. For instance, the hydrophobic defect sites created during the growth or the workup of CNTs were favorable to the adsorption and the electrochemical response of hydrophobic analytes, whereas the hydrophilic oxygen-containing groups produced by acid treatment facilitated the stable adsorption and the direct electrochemistry of redox proteins.

Direct electron-transfer of native hemoglobin in blood: kinetics and catalysis.

A novel approach that uses nature biological tissues, fish blood, for the study of the direct electron-transfer of hemoglobin and its catalytic activity for H(2)O(2) and NO(2)(-) is observed. The direct electron-transfer of hemoglobin in red blood cells in fish blood on glassy carbon electrode was observed for the first time. By simply casting fish blood on GC electrode surface and being air-dried, a pair of well-defined redox peaks for HbFe (III)/HbFe (II) appeared at about -0.36 V (vs SCE) at the fish blood film modified GCE in a pH 7.0 phosphate buffer solution. Ultraviolet visible (UV/VIS) characterization and the enhancement of the redox response of Hb by adding pure Hb in fish blood suggested that Hb preserved the native second structures in the fish blood film. Optical micrographs showed that the RBCs retained its integrity in blood. Hb in blood/GCE maintained its activity and could be used to electrocatalyze the reduction H(2)O(2) and NO(2)(-).

Direct electron-transfer of myoglobin within a new zwitterionic gemini surfactant film and its analytical application for H2O2 detection.

The direct electron-transfer of myoglobin in a new zwitterionic gemini surfactant film with glassy carbon electrode surface has been investigated. A pair of well-defined and quasi-reversible voltammetric peaks was observed at -0.34 and -0.30 V due to the direct electron-transfer of the redox couple of Mb (Fe(III)/Fe(II)). The voltammetric responses of myoglobin-surfactant film under different pH and scan rate conditions were obtained. The presence of hydrogen peroxide changed the typical electrochemical behaviors in terms of bioelectrocatalysis of myoglobin to hydrogen peroxide, and a higher sensitive electroanalytical method for the determination of hydrogen peroxide has been developed.

Direct electron transfer of xanthine oxidase and its catalytic reduction to nitrate.

This work reports on the direct electrochemistry of the xanthine oxidase (XO) from buttermilk, a mononuclear molybdenum enzyme that comprises four redox active cofactors: a five-coordinate mononuclear Mo ion, two [2Fe-2S] clusters, and a flavin adenine dinucleotide (FAD) group. The Mo, [2Fe-2S] and FAD redox responses are obtained from the enzyme immobilized on an activated single-wall carbon nanotubes (SWNTs) modified glassy carbon electrode using protein film voltammetry. The formal potentials of which are -0.61 V, -0.47 V and -0.37 V (vs. SCE) at pH 5.0, respectively. Upon addition of nitrate to the electrochemical cell, a steady-state voltammogram and i-t amprometric response were observed, indicating XO can catalyze the reduction of nitrate.

Direct electron transfer of hemoglobin founded on electron tunneling of CTAB monolayer.

Direct electron transfer and stable adsorption of hemoglobin (Hb) on a carbon paste (CP) electrode were achieved with the aid of a single-chain cationic surfactant, namely, cetyltrimethylammonium bromide (CTAB). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) indicated that CTAB could form a complete monolayer with a high density of positive charges on the surface of the CP electrode, which strongly adsorbed negatively charged protein molecules via electrostatic interactions. The surfactant molecules anchored the protein molecules to align in suitable orientations and acted as electron-tunneling pathways between the protein molecules and the CP electrode. The bioelectrocatalytic activity of the immobilized Hb was confirmed by RAIR and UV-vis spectroscopies, and rapid electrochemical responses to the reduction of oxygen (O2), hydrogen peroxide (H2O2), and nitrite (NO2-) were also obtained.

Enhanced electron transfer for hemoglobin entrapped in a cationic gemini surfactant films on electrode and the fabrication of nitric oxide biosensor.

The direct electrical communication between hemoglobin (Hb) and GCE surface was achieved based on the immobilization of Hb in a cationic gemini surfactant film and characterized by electrochemical techniques. The cyclic voltammograms showed that direct electron transfer between Hb and electrode surface was obviously promoted and then a novel unmediated nitric oxide (NO) biosensor was constructed in view of this protein-based electrode. This modified electrode showed an enzyme-like activity towards the reduction of NO and its amperometric response to NO was well-behaved with a rapid response time and displaying Michaelis-Menten kinetics with a calculated Km(app) value of 84.37 micromol L(-1). The detection limit was estimated to be 2.00 x 10(-8)mol L(-1). This biosensor was behaving as expected that it had a good stability and reproducibility, a higher sensitivity and selectivity and should has a potential application in monitoring NO released from biologic samples.