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A flexible free-standing electrochemical biosensor to detect carcinoembryonic antigen (CEA) is described based on a conducting polypyrrole (PPy) nanocomposite film electrode. The conducting PPy composite was constructed by the sandwiched structure formed by PPy doped with pentaerythritol ethoxylate (PEE) and 2-naphthalene sulfonate (2-NS-PPy) separately via electropolymerization. Gold nanoparticles (AuNPs) were fixed on the PPy composite film by electrodeposition and then connected to CEA aptamer through self-assembly to construct a free-standing electrochemical biosensor breaking away from additional soft substrates and current collector. This PPy composite film-based electrochemical biosensor exhibits satisfying sensing performance for CEA detection, with a linear range from 10−10 g/mL to 10−6 g/mL and a detection limit of 0.033 ng/mL, good specificity and long-term sensing stability (96.8% of the original signal after 15 days). The biosensor also presents acceptable reproducibility with 1.7% relative standard deviation. Moreover, this electrochemical biosensor owns the deformation stability that could bear various deformations (twisting, folding, and knotting) without affecting device’s sensing performance. It can even maintain 99.4% of the original signal under 25% strain deformation. Due to the superior sensing performance, high stability (mechanical deformation and long-term storage), and flexibility, this free-standing electrochemical biosensor proves huge potential in application of flexible and wearable electronics.

A surface-enhanced Raman scattering (SERS) method is proposed for the assay of microRNA 122 based on configuration change of DNA tetrahedron. Firstly, a DNA tetrahedron was self-assembled with one vertex labeled with toluidine blue (TB). Then, it was immobilized on the porous Ni/SiO2@PEI@Au as a SERS platform, which was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). At this time, the DNA tetrahedron was contracted; so, the TB is close to AuNPs and the Raman signal is high. When target microRNA 122 existed, with the nicking enzyme amplification strategy, a great deal of DNA signal chains (S5) was obtained, which can extend the contracted DNA tetrahedron and change it into a three-dimensional DNA tetrahedron. In this case, the TB was far from AuNPs, resulting in a lower Raman signal. Due to the configuration change of DNA tetrahedron, the Raman signal at 1624 cm−1 (with the excitation wavelength of 633 nm) has a linear relationship with the logarithm concentration of microRNA 122. This SERS assay has high sensitivity for microRNA 122 with a determination range from 0.01 aM to 10 fM and a detection limit of 0.009 aM. The recoveries from spiked samples were in the range 95 to 109%. This SERS strategy is designed based on the target-triggered configuration change of DNA tetrahedron, which can give new insight for DNA structures in bioanalysis.

The authors have prepared SnSe nanosheets by a solvothermal method and used them to modify a gold electrode to obtain a photoelectrochemical (PEC) sensor for dopamine (DA) which yields a signal upon irradiation with visible light. On addition of DA, the photocurrent is significantly higher than in the absence of DA, and it increases with increasing DA concentration. A PEC sensor was developed based on this finding which responds linearly to DA in the 0.01 μM to 10 μM concentration range and with a 3 nM detection limit. The application of this PEC sensor in actual samples was also tested. The mechanism of PEC analysis of DA is provided. The electrons of SnSe NSs were excited under irradiation with visible light and transformed from valance band to conduction band. DA as electron donor provided the electrons to SnSe NSs. An enhanced photocurrent was obtained.

A conducting polymer composite was prepared from nano-sized hydroxyaptite (nHAp) doped into poly(3,4-ethylenedioxythiophene) (PEDOT) and then electrodeposited on a glassy carbon electrode (GCE). The nHAp carries carboxy groups and therefore is negatively charged at moderate pH value. When doped into PEDOT (PEDOT-nHAp), it forms a uniform and stable film that exhibits low electrochemical impedance, a large specific surface, and high activity toward the electrochemical oxidation of nitrite. Under optimized conditions and at a relatively low working potential of 0.78 V (vs. SCE), the modified GCE exhibited a linear amperometric response in the 0.25 μM to 1.05 mM nitrite concentration range, and the limit of detection is as low as 83 nM.

An electrochemical sensor for paracetamol is described that consists of a glassy carbon electrode (GCE) that was modified with the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with MnO2 nanoflowers. The hydrothermally synthesized MnO2 nanoflowers possess a large surface area and can be doped into PEDOT through electrochemical deposition to form a conducting polymer nanocomposite. The nanoflowers are shown to be uniformly distributed within the nanocomposite as revealed by elemental mapping analysis. The nanocomposite displays excellent catalytic activity toward the electrochemical oxidation of paracetamol. The modified GCE, best operated at a working potential of around 0.37 V (vs. SCE) has a linear response in 0.06 to 435 μM paracetamol concentration range and a very low limit of detection (31 nM at a signal-to-noise ratio of 3). The sensor exhibits excellent reproducibility and stability, and satisfying accuracy for paracetamol detection in pharmaceutical samples.

The demand of precise assay of nucleic acids and other bioanalytes has been increasing enormously in various areas including point-of-care diagnostics, military, environmental monitoring and so on. Compared with other nucleic acid biosensors, the electrochemical nucleic acid biosensors possess a range of merits like amenable miniaturization, low costs and high sensitivity. Ratiometric electrochemical nucleic acid biosensors can overcome the inherent systematic errors of conventional electrochemical biosensors and enhance the reproducibility and credibility. This short review (with 81 refs.) summarizes the evolvements made in the area of nucleic acid-based biosensors based on ratiometric (electrochemiluminescent, electrochemical and photoelectrochemical) readout in the past few years. Many of the methods discussed here are based on the use of advanced nanomaterials such as quantum dots, graphitic carbon nitrides, graphene oxide, C-dots, gold nanoparticles, metal-organic frameworks, and respective nanohybrids. Three sections (on electrochemiluminescence, classical electrochemical and emerging photoelectrochemical systems) demonstrate the merits of ratiometric assays in various applications. The review ends with a section with conclusions and a discussion of future perspectives. Graphical abstract Ratiometric sensing strategies overcome the intrinsic systematic errors of conventional electrochemical sensors that suffer from environmental and personal factors, and thus leads to remarkably enhanced reproducibility and reliability.

A novel perylene diimide (PDI) functionalized CeO2 nanocomposite (NC) was successfully fabricated via one-pot hydrothermal method. Compared with pure CeO2 nanoparticles (CeO2 NPs), the NC catalyst presents more Ce3+ and active oxygen species and exhibits a higher peroxidase mimicking activity towards the oxidation 3,3′,5,5′-tetramethylbenzidine by H2O2 to form a blue product with an absorption maximum at 652 nm. The composite catalyst shows high sensitivity and selectivity toward H2O2 determination in the range of 20 to 80 μM with a limit of detection (LOD) of 2.59 μM. Based on the colorimetric method, a sensitive method for detecting the reduced glutathione (GSH) was also established over arrange of 1~4 μM with a LOD of 0.92 μM. Electron spin resonance (ESR) experiments suggest that the active radicals during the catalytic processes are •OH and •O2−. A possible synergistic catalytic mechanism is discussed.

An electrochemical sensor that can resist biofouling even when operated in complex biological medium is developed for the determination of dopamine. It is based on the use of the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) that is doped with the water insoluble ionic liquid (IL), 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. A glassy carbon electrode modified with PEDOT/IL is shown to enable accurate determination of dopamine, as a model analyte in the presence of high concentrations of proteins, and resist biological fouling even in native serum. It exhibited a low limit of detection of 33 nM for the detection of dopamine, with a wide linear range from 0.2 to 328 μM (at 0.2 V vs. saturated calomel electrode). The PEDOT/IL modified glassy carbon electrode has a porous microstructure, high electrical conductivity and good stability. The sensor can be used to quantify dopamine in human urine samples with satisfying accuracy. Graphical abstract An antifouling electrochemical sensor capable of detecting target in complex biological samples was developed based on the use of a conducting polymer (PEDOT) that was doped with a water insoluble ionic liquid.

A reagentless and label-free voltammetric modified electrode was developed for the carcinoembryonic antigen (CEA), an important biomarker for colorectal adenocarcinoma. It is based on the use of a reticular hybrid composite consisting of polyaniline (PANI) nanowires grown on the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) that was doped with an ionic liquid (IL). The composite features excellent electrical conductivity and a porous structure, a high specific surface and inherent redox activity. It was placed on a glassy carbon electrode (GCE) and antibody against CEA was immobilized on its surface. The redox current of PANI, measured typically at 0.16 V (vs. Ag/AgCl) serves as the analytical information. This electrode displays linear range that extends from 0.001 to 10 ng mL−1, with a detection limit as low as 0.7 pg mL−1. It also possesses excellent temporal stability and selectivity.

An electrochemical sensor for H2O2 was developed based on electrochemically deposited Prussian blue (PB) nanoparticles doped poly(3,4-ethylenedioxythiophene) (PEDOT). The PEDOT/PB composite was composed of PEDOT wrapped PB nanoparticles, where the conducting polymer PEDOT not only protected the PB particles to warrant high stability, but also connected them to enhance the electron transfer. Owing to the excellent conductivity of PEDOT and unique electrocatalytic activity of PB, the PEDOT/PB modified electrode exhibited good catalytic activity toward the electrochemical reduction of H2O2, and was used for the detection of H2O2 in concentrations ranging from 0.5 to 839 μM, with a detection limit of 0.16 μM. Moreover, the sensor also demonstrated excellent reproducibility, selectivity and long-term stability, showing great promise for the fabrication of electrochemical sensors and H2O2 related biosensors.

A toehold-aided DNA recycling amplification technology was developed based on the combination of toehold-aided DNA recycling and the hemin/G-quadruplex label. The dsDNA formed between aptamer and DNA1 was first immobilized on magnetic beads. On addition of target analyte (exemplified here for riboflavin), the aptamer-riboflavin complex is formed and DNA1 is released by the beads. After magnetic separation, the supernatant containing the released DNA1 is added to a solution containing the hairpin capture DNA on magnetic beads. DNA1 will hybridize with the hairpin capture DNA via toehold binding and branch migration. This process will open the hairpin structure, and an external toehold is formed in the newly formed dsDNA. On addition of reporter DNA containing the G-quadruplex, it will interact with the formed dsDNA via toehold binding and branch migration, resulting in the releasing of DNA1 and capturing of reporter DNA on the magnetic beads. The released DNA1 will bind to another hairpin capture DNA which can start another round of DNA1 recycling. Chemiluminescence (CL) is generated by the G-quadruplex-hemin-luminol CL reaction system. Under optimal conditions, the calibration plot is linear in the 0.1 to 700 nM riboflavin concentration range, with a 30 pM detection limit (at a signal-to-noise ratio of 3). The method was successfully applied to the quantitation of riboflavin in spiked urine samples.

Negatively charged carboxylated nanocrystalline cellulose (CNCC) and positively charged poly(diallyldimethyl ammonium chloride) (PDDA) were alternatingly assembled on the surface of a glassy carbon electrode to form a relatively uniform polyelectrolyte multilayer nanocomposite (CNCC/PDDA)n. It was then incorporated into a matrix of conducting poly(3,4-ethylenedioxythiophene) (PEDOT) electrodeposited on the surface of the electrode. The nanocomposites were prepared in various ratios of PEDOT and (CNCC/PDDA), and then characterized by transmission electron microscopy, scanning electron microscopy, electrochemical impedance spectroscopy, cyclic voltammetry, and differential pulse voltammetry. The PEDOT/(CNCC/PDDA)4 nanocomposite showed the lowest electrochemical impedance and best electrocatalytic activity towards the oxidation of nitrite. Based on these findings, an amperometric sensor was developed which, if operated at 0.80 V (vs. SCE), can detect nitrite in the 0.2 μM to 1.73 mM concentration range with a 57 nM detection limit.

Near infrared (NIR) emitting semiconductor quantum dots can be excellent fluorescent nanoprobes, but the poor biodegradability and potential toxicity limits their application. The authors describe a fluorescent system composed of graphene quantum dots (GQDs) as NIR emitters, and novel MnO2 nanoflowers as the fluorescence quenchers. The system is shown to be an activatable and biodegradable fluorescent nanoprobe for the “turn-on” detection of intracellular glutathione (GSH). The MnO2-GQDs nanoprobe is obtained by adsorbing GQDs onto the surface of MnO2 nanoflowers through electrostatic interaction. This results in the quenching of the NIR fluorescence of the GQDs. In the presence of GSH, the MnO2-GQDs nanoprobe is degraded and releases Mn2+ and free GQDs, respectively. This gives rise to increased fluorescence. The nanoprobe displays high sensitivity to GSH and with a 2.8 μM detection limit. It integrates the advantages of NIR fluorescence and biodegradability, selectivity, biocompatibility and membrane permeability. All this makes it a promising fluorescent nanoprobe for GSH and for cellular imaging of GSH as shown here for the case of MCF-7 cancer cells.

A method is described for rapid (1 h) synthesis of gold nanoclusters (AuNCs) co-functionalized with serine and 11-mercaptoundecanoic acid. The co-functionalized AuNCs exhibit good stability towards temperature, pH values, and over time. They were characterized by atomic force microscopy, high-resolution transmission electron microscopy, dynamic light scattering, and by fluorescence, IR and X-ray photoelectron spectroscopies. The fluorescence of the AuNCs is quenched by Hg(II) and restored on subsequent addition of aureomycin (CTC) or oxytetracycline (OTC). A fluorescent turn-on assay was worked out for simultaneous detection of CTC and OTC based on recording the change of the restored fluorescence measured at 420 and 500 nm under 340 nm photoexcitation. The detection limits are 20 and 9 nM for CTC and OTC, respectively. The concentrations of CTC and OTC can also be visualized by UV illumination. The nanoprobe was successfully applied to the simultaneous determination of CTC and OTC in spiked human urine. Graphical abstract Schematic of a two-wavelength fluorescence recovery method for the simultaneous determination of aureomycin and oxytetracycline. It is based on the use of gold nanocrystals modified with serine and 11-mercaptoundecanoic acid, and on recording the change of the restored fluorescence measured at 420 and 500 nm.

The work describes the synthesis of a nanocomposite consisting of poly(3,4-ethylenedioxythiophene) (PEDOT) doped with gold nanoparticles (AuNPs) by electropolymerization of EDOT in a solution containing colloidal AuNPs. The nanocomposite possesses good conductivity and a 3D microporous network structure owing to the presence of AuNPs. These act as both conductive dopant and template for the growth of the microporous polymer. A glassy carbon electrode modified with the nanocomposite exhibits excellent electrocatalytic activity towards oxidation of nitrite. Best operated at a voltage of 0.8 V (vs. Ag/AgCl), it is capable of detecting nitrite in the 0.2 to 1400 μM concentration range, with a detection limit of 60 nM.

We describe a biosensor for dopamine that is based on the use of a gold electrode modified with carbon nanoparticles (CNPs) coupled to thionine labeled gold nanoparticles (AuNPs) acting as signal amplifiers. The biosensor was constructed by first modifying the CNPs on the gold electrode and adsorbing the thionine on the surface of the AuNPs, and then linking the complementary strand of the dopamine aptamer to the AuNPs via gold-thiol chemistry. Next, dopamine aptamer is added and the duplex is formed on the surface. On addition of a sample containing dopamine, it will interact with aptamer and cause the release of the electrochemical probe which then will be adsorbed on the surface of the CNP-modified gold electrode and detected by differential pulse voltammetry. The current is linearly related to the concentration of dopamine in the 30 nM to 6.0 μM ranges. The detection limit is as low as 10 nM, and the RSD is 3.1 % at a 0.3 μM level (for n = 11). The protocol was successfully applied to the determination of dopamine in spiked human urine samples. We perceive that this method holds promise as a widely applicable platform for aptamer-based electrochemical detection of small molecules.

We describe a highly sensitive and selective peptide-based biosensor for the prostate-specific antigen (PSA). The biotinylated peptide biotin-EHSSKLQKC served as a molecular recognition element. It was self-assembled on the surface of the wells of a 96-well microtiter plate modified with gold nanoparticles (AuNPs). In parallel, streptavidin-coated magnetic beads (strep-MBs) were modified with the enzyme invertase and then added to the peptide-modified wells upon which the modified strep-MBs bind to the peptide via biotin/streptavidin interaction. If a sample containing PSA is placed in the well, PSA will cause the cleavage of the peptide, and a respective quantity of invertase-modified strep-MBs will be released. The supernatant containing the invertase-modified strep-MBs is taken out and sucrose is added which is enzymatically cleaved by invertase. The concentration of the glucose formed after 1 h is quantified with a personal glucose meter. It is linearly related to the concentration of PSA in the range from 80 pg to 7 ng∙mL−1. The detection limit is 30 pg∙mL−1 and the relative standard deviation is 3.7 % (at a level of 500 pg∙mL−1 and for n = 7). The method was successfully applied to the determination of PSA in spiked real human urine. Due to its simplicity, sensitivity and selectivity, this bioassay offers a promising approach to the detection of PSA and other biomolecules.

An electrochemical dopamine sensor was developed by electrochemical modification of a carbon paste electrode with a biocomposite made from poly(3,4-ethylenedioxythiophene) (PEDOT) that was doped with double stranded or single stranded DNA. The resulting conducting biocomposite displays excellent electrocatalytic activity towards the oxidation of dopamine (DA) which is attributed to the presence of the DNA dopant. Under optimum conditions and at a working potential of 300 mV, the modified electrode is capable of detecting DA in the 0.25 to 66 μM concentration range. The detection limit is 74 nM. The sensor is not interfered by uric acid and ascorbic acid.

We have developed a novel resonance light scattering (RLS) assay for the sensitive and selective determination of iodide. It is based on the use of histidine-stabilized gold nanoclusters (His-AuNCs) which undergo fusion and aggregation in the presence of iodide. The resulting enhancement in the intensity of RLS is proportional to the concentration of iodide in the 0.01 to 8.0 μM range, and the detection limit is as low as 1.8 nM at a signal-to-noise ratio of 3. This “turn-on” method is highly selective for iodide and not interfered by other ions commonly present. It was applied to the determination of iodide in (spiked) real water samples.

A sensitive and selective electrochemical sensor for the determination of nitrobenzene (NB) was developed based on a carbon paste electrode (CPE) modified with a nanocomposite prepared from the conducting polymer poly(3,4-ethylenedioxythiophene) and carbon nanotubes. The modified CPE exhibits good conductivity, a large surface area, and excellent catalytic activity towards the electrochemical reduction of NB. Under optimal conditions, the modified CPE is capable of detecting NB in the 0.25 to 43 μM concentration range and with a detection limit at 83 nM. Moreover, the sensor is highly stable and reusable, and free of interferences by other commonly present nitro compounds. It was used to determine NB in wastewater samples.

We have developed a sensitive chemiluminescent (CL) assay for cysteine. It is based on the use of water-soluble and fluorescent silver nanoclusters (Ag NCs) which are found to be able to strongly enhance the weak CL signal resulting from the redox reaction between Ce(IV) ion and sulfite ion. This enhancement is inhibited by cysteine under appropriate conditions. Taking advantage of this specific CL inhibition, a novel CL method for the sensitive and selective detection of cysteine was developed. This effect is interpreted in terms of an electronic energy transfer from excited state intermediate sulfur dioxide (originating from the CL reaction between Ce(IV) and sulfite ions) to the Ag-NCs. The latter become electronically excited and thus can act as a new source of emission. The method was applied to the determination of cysteine in the range from 5.0 nM to 1.0 μM, with a detection limit at 2.5 nM (S/N = 3).

We describe an aptasensor for the determination of dopamine in human serum and with ultrahigh sensitivity and selectivity. The sensor is based on a nanocomposite consisting of reduced graphene oxide (rGO) and the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT). The PEDOT/rGO interface was prepared by electrochemical polymerization of EDOT using graphene oxide as the dopant which is later electrochemically reduced to form rGO. Subsequent covalent modification of the high surface area composite with a selective aptamer enables highly sensitive and selective detection by differential pulse voltammetry. The calibration plot established at a working voltage of 160 mV displays a linear response in the 1 pM to 160 nM concentration range and an unprecedented detection limit of 78 fM. The sensor is fairly selective in not responding to common interferents, and is reusable after regeneration with a 7 M solution of urea. It was successfully applied to (spiked) serum samples and gave recoveries ranging from 98.3 to 100.7 %.