Graphene nanodisks with good conductivity and plenty of edge sites were synthesized to load glucose oxidase (GRD-GOD) and coupled with a Mn2+ doped CdS quantum dot (QD) modified TiO2 electrode (CdS:Mn/TiO2) for a highly sensitive photoelectrochemical (PEC) immunoassay. The specific immune-recognition behaviour can bring the GRD-GOD labelled antigen into the antibody immobilized CdS:Mn/TiO2 interface and dramatically enhance the photocurrent response via a dual signal amplification strategy. First, graphene nanodisks with a strong electron transfer capacity can improve the conductivity of both the insulating protein layers and the CdS:Mn/TiO2 matrix, thus facilitating the regeneration of trapped carriers and hot electrons in the CdS:Mn QD films and enhancing the PEC performance. Second, graphene nanodisks introduce a great number of GOD molecules into a PEC detection process, which catalyze glucose to produce numerous molecules of H2O2. The latter act as sacrificial electron donors to scavenge photogenerated holes, retard the electron–hole recombination, and significantly improve the photo-to-electron conversion efficiency. Based on the dual signal amplification strategy and using a carcinoembryonic antigen as a model target, a highly sensitive PEC immunoassay was therefore developed with an extremely low limit of detection of 5.65 fg mL−1 and a rather wide linear range from 10 fg mL−1 to 1 ng mL−1. The immunoassay showed good reproducibility and stability, as well as good selectivity and high accuracy in serum sample analysis. In this regard, PEC immunosensors may have great application potential for the screening of tumor markers and the prevention and monitoring of serious diseases.

•An AuNPs based colorimetric method is developed for organophosphous pesticide analysis.•The detection mechanism is based on dissolution of AuNPs and AChE hydrolysis reaction.•The colorimetric method has high sensitivity and good stability.•The colorimetric method could be applied in aqueous solutions with high salinity.A simple and sensitive colorimetric method is developed for organophosphrous pesticides (OPs) analysis based on the enzymatic hydrolysis reaction of acetylcholinesterase and the dissolution of gold nanoparticles (AuNPs) in Au3+-cetyltrimethylammonium bromide (Au3+-CTAB) solution. In the absence of OPs, the enzymatic hydrolysis product, thiocholine, would reduce Au3+ and protect the AuNPs from dissolution by the Au3+-CTAB. In the presence of OPs, however, the activity of AChE is inhibited, which could not or could only produce a small amount of thiocholine to consume the Au3+. In this case, the large amount of residual Au3+ dissolves the AuNPs and decreases both the concentration and size of the AuNPs, thus leading to an obvious red-to-light pink or red-to-colorless color change. Under optimal conditions, the colorimetric method could indicate the presence of OPs, with the concentration down to 0.7 ppb. After loading AuNPs on a cellulose paper, an AuNPs-coated dipstick is developed for the detection of OPs, which is highly sensitive with an observable limit of detection of 35 ppb. Practical sample analyses in tap water, apple washing solution, and sea water indicate the colorimetric method has acceptable accuracy and good stability, even in the system containing 5 M of NaCl. Regarding these merits, the as-proposed method could be applied for the point-of-care analysis of OPs in complex systems.

•An enzyme-free electrochemical immunosensor is reported for detecting proteins.•A silver nanocluster/graphene oxide composite is synthesized as nanotag.•The nanotags exhibit highly catalytic activity to the electro-reduction of H2O2.•The as-fabricated immunosensor could detect protein in serum samples.Silver nanoclusters and graphene oxide nanocomposite (AgNCs/GRO) is synthesized and functionalized with detection antibody for highly sensitive electrochemical sensing of carcinoembryonic antigen (CEA), a model tumor marker involved in many cancers. AgNCs with large surface area and abundant amount of low-coordinated sites are synthesized with DNA as template and exhibit high catalytic activity towards the electrochemical reduction of H2O2. GRO is employed to assemble with AgNCs because it has large specific surface area, super electronic conductivity and strong π-π stacking interaction with the hydrophobic bases of DNA, which can further improve the catalytic ability of the AgNCs. Using AgNCs/GRO as signal amplification tag, an enzyme-free electrochemical immunosensing protocol is designed for the highly sensitive detection of CEA on the capture antibody functionalized immunosensing interface. Under optimal conditions, the designed immunosensor exhibits a wide linear range from 0.1 pg mL−1 to 100 ng mL−1 and a low limit of detection of 0.037 pg mL−1. Practical sample analysis reveals the sensor has good accuracy and reproducibility, indicating the great application prospective of the AgNCs/GRO in fabricating highly sensitive immunosensors, which can be extended to the detection of various kinds of low abundance disease related proteins.

In this contribution, a very simple and reliable strategy based on the easy modification of a glassy carbon electrode (GCE) by pre-electrolyzing GCE in ammonium carbamate aqueous solution was employed for the simultaneous determination of hydroquinone (HQ) and catechol (CC). Compared with bare GCE, the incorporation of nitrogen into the GCE surface structure improved the electrocatalytic properties of GCE towards the electro-oxidation of HQ and CC. The nitrogen-introduced GCE (N-GCE) was evaluated for the simultaneous detection of HQ and CC and the linear ranges for HQ and CC were both from 5 to 260 μM. Their detection limits were both evaluated to be 0.2 μM (S/N = 3). The present method was applied for the determination of HQ and CC in real river water samples with recoveries of 95.0–102.1%. In addition, a possible detection mechanism of HQ and CC was discussed.

In this study, we demonstrate a sensitive nitrite detection method using a simple electrochemically aminated glassy carbon electrode (AGCE). The AGCE exhibits a much enhanced sensitivity and good electrochemical response towards nitrite oxidation over a linear range from 0.08 to 1010 μM with a detection limit of 0.01 μM. The validity of the AGCE for real samples was evaluated by determining nitrite in river water and rain water. The good electrochemical performance, construction simplicity and cost-effective functional process of the glassy carbon electrode make the proposed method promising for sensing applications for environmental monitoring. A possible mechanism for the electrocatalytic oxidation process of nitrite is suggested and the kinetics of the catalytic oxidation of nitrite were investigated.

•A label-free and ratiometric aptasensor was fabricated for cocaine detection.•The aptasensor was composed of a non-labeled aptamer and two fluorophores, ATMND and SYBR Green I.•The detection principle was based on a cocaine mediated ATMND displacement reaction.•The ratiometric aptasensor showed high sensitivity and good precision in buffer and body fluids.A label-free ratiometric fluorescence aptasensor has been developed for the rapid and sensitive detection of cocaine in complex biofluids. The fluorescent aptasensor is composed of a non-labeled GC-38 cocaine aptamer which serves as a basic sensing unit and two fluorophores, 2-amino-5,6,7-trimethyl-1,8-naphthyridine (ATMND) and SYBR Green I (SGI) which serves as a signal reporter and a build-in reference, respectively. The detection principle is based on a specific cocaine mediated ATMND displacement reaction and the corresponding change in the fluorescence ratio of ATMND to SGI. Due to the high affinity of the non-labeled aptamer, the good precision originated from the ratiometric method, and the good fluorescence quantum yield of the fluorophore, the aptasensor shows good analytical performance with respect to cocaine detection. Under optimal conditions, the aptasensor shows a linear range of 0.10–10 μM and a low limit of detection of 56 nM, with a fast response of 20 s. The low limit of detection is comparable to most of the fluorescent aptasensors with signal amplification strategies and much lower than all of the unamplified cocaine aptasensors. Practical sample analysis in a series of complex biofluids, including urine, saliva and serum, also indicates the good precision, stability, and high sensitivity of the aptasensor, which may have great potential for the point-of-care screening of cocaine in complex biofluids.

A novel triphenylamine-based organic dye, TTA, with an acrylic group is designed to graft TiO2 nanoparticles for sensitive and selective photoelectrochemical sensing. The synthesized TTA possesses a high molar absorption coefficient, leading to an enhanced photoelectron emission ability of the electron donor. The carboxyl group of TTA acts as not only an electron acceptor but also a linker to connect TTA to TiO2 nanoparticles. Under irradiation, TTA shows fast intramolecular charge transfer from triphenylamine to carboxyl group via the π-bridge of thiophene moiety, thus producing a sensitive photocurrent response. Meanwhile, the acrylic moiety provides an active site for the Michael addition reaction, which would destroy the π-bridge and decrease the photocurrent response. Thus, a selective photoelectrochemical sensing strategy is proposed for detection of small biomolecules. Using cysteine as a model analyte, this sensing strategy shows a detectable range from 1 to 200 μM, without the interference from natural amino acids and various biological reducing reagents. This work offers a new photoelectrochemical route to highly selective and sensitive detection of biologically important small molecules.

•An ultrasensitive enzyme-free electrochemical immunosensor is developed.•A double strand DNA@Au nanoparticle tag is employed for signal amplification.•The tag is able to load a high amount of RuHex via electrostatic interaction.•The immunosensor is of high sensitivity and selectivity for CEA detection.An ultrasensitive enzyme-free electrochemical immunoassay was developed for detection of the fg/mL level carcinoembryonic antigen (CEA) by using a double strand DNA@Au nanoparticle (dsDNA@AuNP) tag and hexaammineruthenium(III) chloride (RuHex) as the electroactive indicator. The dsDNA@AuNP was synthesized by one-pot hybrid polymerization of dsDNA on initiator DNA modified AuNPs via hybridization chain reaction. The immunosensor was prepared by covalently cross-linking capture antibody on chitosan/AuNP nanocomposite modified glass carbon electrode. The AuNPs accelerated the electron transfer and led to high detection sensitivity. With a sandwich-type immunoreaction and a biotin–streptavidin affinity reaction, the dsDNA@AuNP tag was conjugated on the immunocomplex to bring a high amount of RuHex to the electrode surface via electrostatic interaction, resulting in an amplified electrochemical signal. Under optimal conditions, the proposed sensing platform showed a wide linear detection range from 10 fg/mL to 10 ng/mL along with a detection limit of 3.2 fg/mL for CEA. The immunosensor exhibited high sensitivity and good stability, showing a promising application in early cancer diagnosis and could be extended to sensitive electrochemical biosensing of other analytes.An ultrasensitive enzyme-free electrochemical immunoassay was developed for detection of fg/mL level carcinoembryonic antigen by using a double strand DNA@Au nanoparticle (dsDNA@AuNP) tag and hexaammineruthenium(III) chloride (RuHex) as the electroactive indicator.

Selective detection of cysteine in serum samples was achieved on a graphene nanoribbon (GNR) and Nafion nanocomposite modified electrode with high precision. The superior conductivity and abundant amount of active chemical oxygen groups on the edge of GNR led to extremely highly electrocatalytic activity of GNR towards the electrochemical oxidation of cysteine at +0.025 V. The electrocatalytic behavior was further used for sensitive detection of cysteine by differential pulse voltammetry. Under optimized conditions, the calibration curve was linear in the range from 25 nM to 500 μM. The electrochemical sensor showed strong antifouling ability, good stability and selectivity. It could effectively exclude the interferences from other kinds of biothiols and the biological relevant species, thus had great perspective for in vivo analysis of biological samples.Highlights► Graphene nanoribbon and Nafion nanocomposite was used for the sensitive electrochemical detection of cysteine. ► The sensor could discriminate cysteine from other bio-relevant reagents and other biothiols. ► The good selectivity and high sensitivity was due to the co-interaction of the GNR and the negatively charged Nafion.

A new protocol is proposed for magnetic loading and sensitive electrochemical detection of phenol via the tyrosinase cross-linked mesoporous magnetic core/shell microspheres. The mesoporous magnetic microspheres, characterized by transmission electron microscopy, N2 adsorption/desorption isotherms, and magnetic curve displays high capacity for enzyme immobilization and strong magnetism to adhere to the magnetic electrode surface without any additional adhesive reagent. The biosensor exhibits a wide linear response to phenol ranging from 1.0 × 10−9 to 1.0 × 10−5 M, a high sensitivity of 78 μA mM−1, a low detection limit of 1 nM, and a fast response rate (less than 5 s). The proposed method is simple, rapid, inexpensive and convenient in electrode renewal, which is recommended as a promising experimental platform for wider applications in biosensing.

Electrochemical reduced β-cyclodextrin dispersed graphene (β-CD-graphene) was developed as a sorbent for the preconcentration and electrochemical sensing of methyl parathion (MP), a representative nitroaromatic organophosphate pesticide with good redox activity. Benefited from the ultra-large surface area, large delocalized π-electron system and the superconductivity of β-CD-graphene, large amount of MP could be extracted on β-CD-graphene modified electrode via strong π–π interaction and exhibited fast accumulation and electron transfer rate. Combined with differential pulse voltammetric analysis, the sensor shows ultra-high sensitivity, good selectivity and fast response. The limit of detection of 0.05 ppb is more than 10 times lower than those obtained from other sorbent based sensors. The method may open up a new possibility for the widespread use of electrochemical sensors for monitoring of ultra-trace OPs.Graphical abstractHighlights► An electrochemical sensor is fabricated based on β-CD dispersed graphene. ► The sensor could selectively detect organophosphate pesticide with high sensitivity. ► The β-CD dispersed graphene owns large adsorption capacity for MP and superconductivity. ► The β-CD dispersed graphene is superior to most of the porous sorbents ever known.

An ultrasensitive amperometric acetylcholinesterase (AChE) biosensor was fabricated by controlled immobilization of AChE on gold nanoparticles/poly(dimethyldiallylammonium chloride) protected Prussian blue (Au–PDDA–PB) nanocomposite modified electrode surface for the detection of organophorous pesticide. The Au–PDDA–PB membrane served as an excellent matrix for the immobilization of enzyme, which not only enhanced electron transfer but also possessed a relatively large surface area. In addition, the surface hydrophilicity of the Au–PDDA–PB nanocomposite was finely controlled in the static water contact angle range of 25.6–78.1° by adjusting the ratio of gold nanoparticles to PDDA–PB. On an optimized hydrophobic surface, the AChE adopts an orientation with both good activity and stability, which has been proven by electrochemical methods. Benefit from the advantages of the Au–PDDA–PB nanocomposite and the good activity and stability of AChE, the biosensor shows significantly improved sensitivity to monocrotophos, a typical highly toxic organophorous pesticide, with wide linear range (1.0–1000 pg/mL and 1.0–10 ng/mL) and an ultra-low detection limit of 0.8 pg/mL. The biosensor exhibits accuracy, good reproducibility and stability. This strategy may therefore provide useful information for the controlled immobilization of protein and the design of highly sensitive biosensors.

A sensitive amperometric acetylcholinesterase (AChE) biosensor was fabricated based on mesocellular silica foam (MSF), which functioned as both an enzyme immobilization matrix and a solid phase extraction (SPE) material for the preconcentration of target molecules. The hydrophilic interface, the good mechanical/chemical stability, and the suitable pore dimension of MSF provided the entrapped AChE a good environment to well maintain its bioactivity at basic condition. The AChE immobilized in MSF showed improved catalytic ability for the hydrolysis of acetylthiocholine, as evidenced by the increasing of the oxidation current of thiocholine, the enzymatic catalytic hydrolysis production of acetylthiocholine. In addition, the MSF with large surface area showed a modest adsorption capacity for monocrotophos, a model organophosphate used in this study, via the hydrogen bond or physical adsorption interaction. The combination of the SPE and the good enzyme immobilization ability in MSF significantly promoted the sensitivity of the biosensor, and the limit of detection has lowered to 0.05 ng/mL. The biosensor exhibited accuracy, good reproducibility, and acceptable stability when used for garlic samples analysis. The strategy may provide a new method to fabricate highly sensitive biosensors for the detection of ultra-trace organophosphorous pesticide infield.