This work designed a nanocomposite to enhance the photoelectrochemical (PEC) efficiency by depositing BiVO4 nanoparticles on TiO2 nanospheres with a solvothermal method. The TiO2–BiVO4 heterostructure was characterized with various spectroscopic and microscopic techniques and was employed as a nanostructured support to cross-link DNA aptamer for constructing a visible-light driven PEC aptasensor. The TiO2 nanospheres provided a biocompatible microenvironment, and the high surface area of the heterostructure enhanced the loading of aptamer molecules. The small energy gap of BiVO4 improved the PEC property of the nanocomposite compared with the pure TiO2 under visible-light irradiation. The advantages of the nanocomposite along with the high loading of recognition molecules greatly improved the sensitivity of the aptasensor. Using 17β-estradiol as an analyst model, the proposed PEC biosensor showed excellent analytical performance with high sensitivity, low detection limit of 0.022 pM, and high selectivity in a detectable concentration range of 0.1–250 pM, indicating the promising application of the designed TiO2–BiVO4 heterostructure in PEC biosensing.Keywords: 17β-estradiol; DNA aptamers; nanobiosensing; photoelectrochemical aptasensor; sensitization; TiO2−BiVO4 heterostructure;

•MoS2 nanosheets were grown on TiO2 nanorods to obtain a heterostructure.•The thickness of MoS2 nanosheets in the composite is much thinner than that of MoS2 microspheres.•Photoelectrochemistry (PEC) of the heterostructure is superior to that of the single component under visible irradiation.•Glucose oxidase (GOx) PEC biosensor was developed based on the heterostructure.•The performance of PEC GOx sensor was evaluatedIn the present work, MoS2 nanosheets were composited with TiO2 nanorods to obtain a heterostructure with improved photoelectrochemical (PEC) performance. TiO2 nanorods were initially prepared using a hydrothermal method, and then MoS2 nanosheets were synthesized using TiO2 nanorods as the substrates. Various spectroscopic and microscopic methods were used to investigate the morphology and composition of the composite. Electrochemical impedance spectroscopy was also conducted to demonstrate the excellent electrical conductivity of the composite. The bioactivity of glucose oxidase (GOx) to glucose was validated with cyclic voltammetry in air-saturated solution. Finally, a PEC biosensor was fabricated by immobilizing GOx on MoS2 nanosheet-TiO2 nanorod composite modified ITO electrode. Owing to the high specific surface area, good conductivity, excellent biocompatibility and small band gap associated with this composite, the PEC glucose biosensor exhibited a dynamic range of 0.1–10.5 mM, sensitivity of 0.81 μA mM−1, and a detection limit of 0.015 mM within the visible spectrum.

In this paper, we have developed a gold nanoparticle (GNP) decorated titanate nanotubes (TNT) nanocomposite that aids in the direct electron transfer of a large enzyme, such as glucose oxidase (GOD), in which the electroactive site of flavin adenine dinucleotide is deeply buried within the enzyme. The ionic liquid, brominated 1-decyl-3-methyl imidazole, was used to immobilise the nanocomposite and the enzyme on a glassy carbon electrode to further aid in the electron transfer between GOD and the electrode surface. Nafion was also added to anchor the biosensor scaffold. Initially, the tubiform geometry of titanate nanomaterials and the GNP-TNT nanocomposite was confirmed by microscopic and spectroscopic techniques before glucose oxidase was entrapped in the nanocomposite. Based on voltammetric results, this biosensor showed a strong electrocatalytic capability towards glucose (with a heterogeneous electron transfer rate constant of 7.1 s−1 at 180 mV s−1) and the calibration for glucose exhibited a high sensitivity (5.1 μA mM−1) and a wide linear range (0.01–1.2 mM). These results demonstrated superior analytical performance of our biosensor over others fabricated using bulkier TiO2 nanoparticles or nanobundles, which could be attributed to a high degree of biocompatibility to glucose oxidase and electrical conductivity of the nanocomposite.

•Aniline was polymerized on TiO2 nanotubes (TNTs) to form PANI–TNT composite.•TEM, SAED, XRD, FT-IR, EIS and UV–vis results confirmed that an intimate PANI–TNT composite was well formed.•A glucose oxidase biosensor was fabricated using the PANI–TNT as scaffold.•Heterogeneous electron transfer rate constant of GOD at PANI–TNT is larger than that at other modified electrodes.•The performance of PANI–TNT modified GOD biosensor is superior over that of other similar biosensors.The significant and novel aspect of this work lies in the preparation of a polyaniline (PANI)–TiO2 nanotube (TNT) composite which can provide excellent biocompatibility, good electrical conductivity, low electrochemical interferences and high signal-to-noise ratio for the development of electrochemical biosensors. Using a hydrothermal method, TiO2 nanoparticles were initially transformed into TNTs, on which aniline was then polymerized by oxidative polymerization to form an intimate and uniform PANI–TNT composite. After being characterized by different spectroscopic techniques, the PANI–TNT composite was used to immobilize glucose oxidase (GOD) for the construction of an electrochemical biosensor. The direct electrochemistry and electro-catalytic performance of the biosensors based on PANI–TNTs and TNTs was studied by cyclic voltammetry. The direct electrochemistry of GOD on PANI–TNTs modified electrodes was achieved and the heterogeneous electron transfer rate constant (ks) for GOD was estimated to be 9.3 s−1. Furthermore, the redox currents of GOD at PANI–TNT modified biosensor were improved by ∼55% compared to that at TNT modified biosensor. The PANI–TNT modified GOD biosensor exhibited good sensitivity (11.4 μA mM−1), wide dynamic range (10–2500 μM) and low limit of detection (0.5 μM), which are attributed to the improved properties of the composite.

•A MWCNT–TiO2 nanotube nanocomposite that complements the individual component.•Improved electrochemical and biocompatible properties of a biosensor scaffold consisting of a MWCNT–TiO2 nanotube nanocomposite.•Enhanced direct electrochemistry and catalytic capability of horseradish peroxidase at an electrochemical biosensor scaffold consisting of a MWCNT–TiO2 nanotube nanocomposite.A significant aspect of this work is the development of a multi-wall carbon nanotube (MWCNT)-titanate nanotube (TNT) nanocomposite to serve as a biocompatible scaffold with high conductivity on a biosensor surface. Unlike other scaffolds consisting of MWCNTs alone or TNTs alone, the MWCNT–TNT nanocomposite synergistically provides excellent biocompatibility, good electrical conductivity, low electrochemical interferences and a high signal-to-noise ratio. For comparison, after characterising a scaffold consisting of MWCNTs alone, TNTs alone and a MWCNT–TNT nanocomposite using several spectroscopic techniques, the analytical performance of a horseradish peroxidase (HRP) electrochemical biosensor was evaluated using cyclic voltammetry and differential pulse voltammetry. The scaffold consisting of MWCNTs alone displayed a high background charging current, a low signal-to-noise ratio and distinct electrochemical interference from its surface functional groups. In contrast, the direct electrochemistry and the catalytic capability of HRP at MWCNT–TNT modified biosensors towards H2O2 was demonstrated to be ~51% and ~144% enhanced, respectively, compared to those at TNT modified biosensors. Meanwhile, MWCNT–TNT nanocomposite modified HRP biosensors also exhibited higher sensitivity (4.42 μA mM−1) than TNT modified HRP biosensors (1.48 μA mM−1). The above superior performance was attributed to the improved properties of MWCNT–TNT nanocomposite as biosensor scaffold compared to its two individual components by complementing each component and synergistically sustaining the characteristic features of each component.

•A composite consisting of graphene nanoplatelets (GNPs) and titanate nanotubes (TNTs) were synthesized.•The composite was characterized by XRD, TEM, FT-IR and impedance.•Horseradish peroxidase (HRP) biosensors were developed using the composite, GNPs and TNTs as immobilization material respectively.•Direct electron transfer of HRP was achieved on the composite biosensor.•The composite HRP biosensor shows superiority over the other two in electrochemical and analytical performance.A novel nanocomposite consisting of graphene nanoplatelets (GNPs) and titanate nanotubes (TNTs) have been synthesized successfully utilizing the hydrothermal method. The GNP–TNT composite was characterized by transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and electrochemical impedance spectroscopy. The voltammetric characterization of GNP–TNT composite, pure GNPs and pure TNTs modified horseradish peroxidase (HRP) biosensors were conducted to select the most suitable electrode immobilization material for enzyme biosensors. The GNPs was firstly eliminated owing to its extremely high background charging current, distinct electrochemical interference from its surface functional groups and low signal to noise ratio. Next, the direct electron transfer of HRP on electrode and the catalytic current of HRP towards H2O2 was increased around 45% and 72% respectively on GNP–TNT composite modified electrodes compared with those on pure TNTs modified electrodes. GNP–TNT composite modified HRP biosensor also exhibited superiority over pure TNTs modified HRP biosensor in the analytical performance. The precision and stability study provided additional evidence for the feasibility of using GNP–TNT composite as electrode modification material.

A copper monolayer was formed on a gold electrode surface via underpotential deposition (UPD) method to construct a Cu UPD|DTBP–Protein G immunosensor for the sensitive detection of 17β-estradiol. Copper UPD monolayer can minimize the non-specific adsorption of biological molecules on the immunosensor surface and enhance the binding efficiency between immunosensor surface and thiolated Protein G. The crosslinker DTBP (Dimethyl 3,3′-dithiobispropionimidate·2HCl) has strong ability to immobilize Protein G molecules on the electrode surface and the immobilized Protein G provides an orientation-controlled binding of antibodies. A monolayer of propanethiol was firstly self-assembled on the gold electrode surface, and a copper monolayer was deposited via UPD on the propanethiol modified electrode. Propanethiol monolayer helps to stabilize the copper monolayer by pushing the formation and stripping potentials of the copper UPD monolayer outside the potential range in which copper monolayer can be damaged easily by oxygen in air. A droplet DTBP–Protein G was then applied on the modified electrode surface followed by the immobilization of estradiol antibody. Finally, a competitive immunoassay was conducted between estradiol–BSA (bovine serum albumin) conjugate and free estradiol for the limited binding sites of estradiol antibody. Square wave voltammetry (SWV) was employed to monitor the electrochemical reduction current of ferrocenemethanol and the SWV current decreased with the increase of estradiol–BSA conjugate concentration at the immunosensor surface. Calibration of immunosensors in waste water samples spiked with 17β-estradiol yielded a linear response up to ∼2200 pg mL−1, a sensitivity of 3.20 μA/pg mL−1 and a detection limit of 12 pg mL−1. The favorable characteristics of the immunosensors such as high selectivity, sensitivity and low detection limit can be attributed to the Cu UPD|DTBP–Protein G scaffold.Highlights► Copper monolayer was formed on gold electrode via underpotential deposition (UPD). ► Copper UPD monolayer minimized non-specific adsorption of biomacromolecules. ► Propanethiol enhanced stability of copper UPD monolayer on gold electrode surface. ► Protein G was thiolated by DTBP to improve binding of antibody on immunosensor. ► SWV was used to monitor conductivity change on immunosensor surface.

The direct electrochemistry of horseradish peroxidase (HRP) on a novel sensing platform modified glassy carbon electrode (GCE) has been achieved. This sensing platform consists of Nafion, hydrophilic room-temperature ionic liquid (RTIL) and Au nanoparticles dotted titanate nanotubes (GNPs-TNTs). The composite of RTIL and GNPs-TNTs was immobilized on the electrode surface through the gelation of a small amount of HRP aqueous solution. The composite was characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD) and infrared spectroscopy (IR). UV–Vis and IR spectroscopy demonstrated that HRP in the composite could retain its native secondary structure and biochemical activity. The HRP-immobilized electrode was investigated by cyclic voltammetry and chronoamperometry. The results from both techniques showed that the direct electron transfer between the nanocomposite modified electrodes and heme in HRP could be realized. The biosensor responded to H2O2 in the linear range from 5 × 10−6 to 1 × 10−3 mol L−1 with a detection limit of 2.1 × 10−6 mol L−1 (based on the S/N = 3).Highlights► Au nanoparticles modified titanate nanotubes (GNPs-TNTs) were synthesized. ► Hydrophilic ionic liquid (RTIL) and GNPs-TNTs consist of a sensing scaffold. ► Horseradish peroxidase aqueous solution was employed as a gelator to gelate RTIL. ► The sensing scaffold was immobilized on electrode surface by gelation. ► Direct electron transfer was achieved on this sensing scaffold modified electrode.