The authors report a dual signal amplification strategy for improving the sensitivity of an electrochemical immunosensor. Single-walled carbon nanotubes (SCNTs) served as supports for aptamers and alkaline phosphatase (ALP), and then the modified SCNTs were utilized as electrochemical probes. The aptamer contains phosphate groups, while ALP can hydrolyse one molecular pyrophosphate into two molecular phosphate ions. The phosphate can react with molybdate to form a redox-active molybdophosphate precipitate on the surface of the glassy carbon electrode (GCE). With applying a relatively low voltage of 0.21 V (vs. Ag/AgCl), the generated electrochemical current intensity is proportional to the concentration of the analyte. The cancer biomarker platelet-derived growth factor BB (PDGF-BB) was chosen as a model antigen (analyte). The immunosensor was prepared by sequential capturing of the antibodies against PDGF-BB, analyte (PDGF-BB) and modified SCNTs on the GCE to form a sandwich structure. The current signal is linear in the concentration range of 0.1 pg mL−1 to 50 ng mL−1 PDGF-BB, with a detection limit as low as 50 fg mL−1. The immunosensor was finally applied in the determination of PDGF-BB concentration in serum samples. Our perception is that this signal amplification strategy can be adapted to the preparation of other immunosensors and will find wide applications in the detection of different biomarkers and related species.

The paper describes a colorimetric sandwich enzyme-linked immunosorbent assay (ELISA) for the determination of human IgG. It is based on the use of an Fe(II) coordination complex as a signal amplifier and of mesoporous silica nanoparticles modified with glucose oxidase (GOx) and secondary antibodies (Ab2). After formation of the immuno sandwich complex, the quantity of GOx is proportional to the quantity of IgG. On addition of Fe(II) and glucose, GOx catalyzes the oxidation of glucose to produce hydrogen peroxide which oxidizes Fe(II) to Fe(III). After adding a stop solution containing the complexing ligand 1,10-phenanthroline (Phen), un-reacted Fe(II) forms an orange-red complex with Phen which can be detected by plate reader and even seen with bare eyes. This sandwich ELISA has a linear response in the 1 pg.mL−1 to 100 ng.mL−1 human IgG concentration range and a 860 fg.mL−1 detection limit. This is 20 times lower than the commercial ELISA for human IgG. The assay also is selective, stable, highly sensitive and cost-effective.

The authors describe a fluorometric assay for determination of the activity and inhibition of alkaline phosphatase (ALP). It is based on the use of silver nanoclusters (AgNCs) templated with dC12 strands and acting as a fluorescent probe. The fluorescence of such AgNCs is initially quenched by the addition of Cu(II) in low concentration, and then recovered by the subsequent addition of pyrophosphate (PPi) due to strong binding between PPi and Cu(II). In the presence of ALP, PPi will be hydrolyzed to form phosphate ion (Pi), and this decreases the concentration of bound Cu(II). As a result, the fluorescence of AgNCs is diminished again. The decrease in fluorescence intensity is proportional to the actual activity of ALP. The assay has a detection limit as low as 0.078 mU·mL−1. The method was applied to the determination of ALP activity in spiked human serum and gave recoveries that ranged from 96.2% to 105.9%. The inhibitory effect of L-phenylalanine on the activity of ALP was also evaluated. The assay is simple and inexpensive. In our perception, it simplifies clinical measurement, thereby improving the diagnosis of ALP-associated diseases and improving the screening for new drugs.

A highly sensitive electrochemical scheme for the detection of the cancer biomarker carcinoembryonic antigen (CEA) is reported. An aptamer-based formation of redox-active molybdophosphate is combined with rolling circle amplification (RCA). Gold nanoparticles (AuNPs) were utilized as supporting matrix for the aptamer and the primer to increase the primer/aptamer loading. The formation of a sandwich structure of anti-CEA/CEA/AuNP − aptamer-primer onto a glassy carbon electrode surface enabled an effective RCA reaction. Square wave voltammetry was applied to record the current signal at a peak potential of 0.18 V vs. Ag/AgCl. Compared to the immunosensor without RCA, the sensitivity of the immunosensor with RCA is increased by about a factor of 6. In the latter case, a wider linear range was obtained for the determination of CEA (from 0.5 pg mL−1 to 1 ng mL−1) with a detection limit as low as 0.1 pg mL−1. The practical applicability of the method was studied by analyzing CEA in human serum samples. The results were found to be in good agreement with those obtained by a reference method. Conceivably, the method can be applied to other DNA amplifications and therefore has the potential of finding wide applications.

The authors describe a strategy for improving the sensitivity of electrochemical immunoassays that is based on the use of hydroxyapatite nanoparticles (HAP-NPs). Signal generation is based on the reaction of phosphate groups present in HAP-NPs with molybdate to form a redox-active molybdophosphate precipitate on the surface of the electrode that facilitates the generation of an electrochemical current. The cancer biomarker α-fetoprotein (AFP) was chosen as a model antigen (analyte). Antibodies against AFP (Ab2) were immobilized on the HAP-NPs to obtain the signal probe. The reaction of HAP-NPs with molybdate and the performance of the immunoelectrode were characterized in detail. Based on this signal amplification approach, an immunoassay was worked out that has a linear range that extends from 0.1 pg·mL−1 to 1 ng·mL−1 and a detection limit as low as 50 fg·mL−1. The assay was successfully applied to the determination of AFP in serum samples. In our perception, this new signal amplification scheme can be adapted to numerous other immunoassays.

In this work, we developed a low-cost and sensitive electrochemical sensor for nonenzymatic detection of glucose based on ball-in-ball hollow ternary nickel cobalt sulfide spheres (NiCo2S4). A facile anion exchange method to synthesize the hollow structure of NiCo2S4 is demonstrated. The synthesized hollow NiCo2S4 was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The electrochemical sensing properties of NiCo2S4 towards glucose were evaluated. Due to the synergetic effects from both Ni and Co species with multiple valence transitions and the high surface area of the hollow NiCo2S4, the nanostructure exhibited good performance towards glucose oxidation. A simple and selective sensing platform for the assay of glucose has been developed in 0.1 M KOH solution at 0.55 V. The sensor displays a wide linear range from 0.005 to 2 mM with a detection limit of 2 μM for glucose detection. For the determination of glucose in serum samples, the results are satisfactory when compared to those obtained from a local hospital.

In this study, we report a sensitive and selective photoelectrochemical sensor for the detection of Cu2+ in human serum samples based on a TiO2/CdS nanorod array. To prepare the sensor, a TiO2 nanorod array was initially deposited onto a fluorine-doped tin oxide (FTO) conductive glass, and then, CdS nanoparticles were deposited onto the TiO2 nanorod array surface. The well-matched energy level between TiO2 nanorod and CdS efficiently suppressed the recombination of photogenerated electron and hole (e−/h+) pairs; this led to improved photon-to-current conversion efficiency. Experimental results demonstrated increased photoelectrochemical current of TiO2/CdS as compared to that of TiO2 alone. Utilizing the interaction between Cu2+ and CdS, the constructed photoelectrochemical sensing platform shows selective response towards Cu2+. Increased Cu2+ ion concentration resulted in the decreased photoelectrochemical current intensity, and a linear range of 1 nM to 1 μM was obtained with a detection limit of 0.5 nM. Due to its good performances, the sensor was successfully applied for the direct detection of Cu2+ in human serum samples, and the results suggested the potential of this sensor for practical clinical applications.

In addition to its primary function as a genetic material, deoxyribonucleic acid (DNA) is also a potential biologic energy source for molecular electronics. For the first time, we demonstrate that DNA can generate a redox electric current. As an example of this new functionality, DNA generated redox current was used for electrochemical detection of human epidermal growth factor receptor 2 (HER2), a clinically important breast cancer biomarker. To induce redox current, the phosphate of the single stranded DNA aptamer backbone was reacted with molybdate to form redox molybdophosphate precipitate and generate an electrochemical current of ∼16.8 μA/μM cm2. This detection of HER2 was performed using a sandwich detection assay. A HER2 specific peptide was immobilized onto a gold electrode surface for capturing HER2 in buffer and serum. The HER2 specific aptamer was used as both ligand to bind the captured HER2 and to generate a redox current signal. When tested for HER2 detection, the electrochemical current generated by the aptasensor was proportional to HER2 concentration in the range of 0.01 to 5 ng/mL, with a current generated in the range of ∼6.37 to 31.8 μA/cm2 in both buffer and serum. This detection level is within the clinically relevant range of HER2 concentrations. This method of electrochemical signal amplification greatly simplifies the signal transduction of aptasensors, broadening their use for HER2 analysis. This novel approach of using the same aptamer as biosensor ligand and as transducer can be universally extended to other aptasensors for a wide array of biodetection applications. Moreover, electric currents generated by DNA or other nucleic acids can be used in molecular electronics or implanted devices for both power generation and measurement of output.

•Silver nanoclusters based assay for Fe (III) was reported.•Fluorescence intensity of silver nanoclusters was pH dependent and pH reversible.•Sensitive assay of Fe (III) was achieved through pH adjustment of silver nanoclusters.•The sensitivity was about 50 times higher than traditional method.•The assay was successfully applied for detection of Fe (III) in human serum samples.Silver nanoclusters (Ag NCs) were synthesized based on dC12 strands as template. The fluorescence intensity of the Ag NCs was found to be pH dependent and pH reversible, that is, the fluorescence intensity was quenched with the decrease of pH and can be recovered to exact value again with the adjustment of pH value to neutral. This pH dependent fluorescence property was utilized for the sensitive detection of Fe (III). With the addition of acid Fe (III) solution to decrease the pH value of Ag NCs and then corresponding amount of alkali to adjust the pH to neutral, the fluorescence intensity of Ag NCs was found to be still quenched in contrary to that without Fe (III). Compared to the direct addition of Fe (III) into Ag NCs, this method utilizing pH adjustment resulted in around 50 times of sensitivity enhancement for Fe (III) detection. At the optimal pH of 4, linear relationship was obtained between fluorescence change and logarithm of Fe (III) concentration in the range from 0.01 to 1 μM with the detection limit of 3 nM. Recovery test was also performed for the detection of Fe (III) in human serum samples with good results.

We reported an enzyme-free ELISA to detect breast cancer biomarker human epidermal growth factor receptor 2 (HER2) in human serum samples. Instead of enzymes (such as horseradish peroxidase) used in traditional ELISA, CuO nanoparticles were utilized as the signal probe. Compared to traditional enzymes, CuO nanoparticles have the advantages of low cost and good stability. After dissolving CuO nanoparticles with acid, the Cu (II) ions generated catalyzed the reaction of o-phenylenediamine with ascorbic acid to produce fluorescent quinoxaline derivative molecules. The immunoassay displays high sensitivity and good selectivity towards HER2 with detection limit as low as 9.65 pg·mL− 1. The assay was successfully applied to the analysis of HER2 in serum of breast cancer patients. The analysis results demonstrated the HER2 level in the serum samples determined by our assay were in good agreement with those determined by commercial HER2 ELISA kit. This enzyme-free ELISA assay can be easily adapted to the detection of other analytes. With these merits, the simple, sensitive and cost effective fluorescence immunoassay shows great potential for clinical applications.

The protease BACE1 (the β-site amyloid precursor protein cleaving enzyme 1) catalyzes the first step in the synthesis of β-amyloids (Aβ), peptides that accumulate in the brain in Alzheimer’s disease (AD). Measurement of BACE1 activity is important for the development of BACE1 inhibitors to slow or stop AD. To measure BACE1 cleavage of the electrode-immobilized substrate peptide, we developed a redox-generating hydroxyapatite (HAP) probe which generates electrochemical current by reaction of the nanoparticle with molybdate (MoO42–). The probe combines alkaline phosphatase (ALP) for dual signal amplification and Aβ antibody to bind the probe to the immobilized peptide substrate on the surface of the electrode. We measured the activity of BACE1 at concentrations ranging from 0.25 to 100 U/mL. The use of the dual-signal HAP–ALP probe increased the signal by an order of magnitude compared to HAP-only probe, enabling detection limits as low as 0.1 U/mL. To measure the inhibition of BACE1 activity, the BACE1 inhibitor OM99-2 was added to 25 U/mL of BACE1 in concentrations ranging from 5 to 150 nM. The observed detection limit of inhibition is 10 nM of OM99-2. These results demonstrate the capabilities of this novel biosensor to measure BACE1 activity and inhibitors of BACE1 activity. To the best of our knowledge this is the first report that reaction of HAP nanoparticles with molybdate can generate electrochemical current. This dual signal amplification strategy can be extended to other electrochemical assays and adapted for wide applications.

A sensitive assay for the detection of biothiols was reported based on biothiol-induced aggregation of silver nanoparticles (Ag NPs). In the presence of biothiols, such as cysteine (Cys), homocysteine (Hcy) and glutathione (GSH), the aggregation of Ag NPs resulted in the colorimetric change of the nanoparticles, which could be observed by the naked eye and measured with UV-vis spectroscopy. More importantly, the method can be developed into a more sensitive and stable electrochemical biothiol sensing method by monitoring the electrochemical current of the Ag NPs. In the electrochemical analysis, the biothiol-induced aggregation of Ag NPs resulted in the decrease of electrochemical current. Experimental results demonstrated that for the detection of GSH, the detection limit of the electrochemical method was as low as 200 nM, which is more than 2 orders of magnitude lower than that of the colorimetric method. The developed method is simple in design and fast in operation, which can be applied to other target-induced aggregation assays and demonstrates wide applications in bio-sensing fields.

•Protein kinase (PKA) and alkaline phosphatase (ALP) are clinically relevant enzymes.•Electrochemical biosensor for assay the activity and inhibition of both PKA and ALP was reported.•The assay is based on reaction of phosphate ions with molybdate to form redox precipitates.•The biosensor was applied for screening enzyme inhibitors.•The biosensor was also used to measure drug-stimulated PKA from lysates of HeLa cells.Protein kinase (PKA) and alkaline phosphatase (ALP) are clinically relevant enzymes for a number of diseases. In this work, we developed a new simple electrochemical biosensor for the detection of the activity and inhibition of both PKA and ALP. One common feature of the PKA and ALP catalyzing process is that PKA can hydrolysis adenosine-5′-triphosphate (ATP) and ALP can hydrolysis pyrophosphate, both reactions produce phosphate ions, and the amount of phosphate ion produced is proportional to enzyme activity. Our assay is based on the principle that phosphate ions react with molybdate to form redox molybdophosphate precipitates on the electrode surface, thus generating electrochemical current. The detection limit for PKA and ALP were much lower than existing assays. The biosensor has good specificity and was used to measure drug-stimulated PKA from lysates of HeLa cells. We also evaluated the use of the biosensor as a screening tool for enzyme inhibitors. To the best of our knowledge, this is the first report of a biosensor capable of detecting the activity of both PKA and ALP. This tool has the potential to simplify PKA and ALP clinical measurement, thereby improving diagnostics of relevant diseases. It also may serve as the basis for a simple screening method for new enzyme inhibitors for disease treatment.

The authors describe a method for highly sensitive and selective determination of the activity of protein kinase (PKA). It is based on the finding that silver nanoclusters (AgNCs) can act as a nucleus to catalyze further deposition of silver nanoparticles. This causes the color of a solution to change from pale yellow to black. In the detection scheme presented here, the substrate peptide is phosphorylated by PKA in the presence of ATP. The resulting phosphopeptides bind to oligonucleotide-stabilized AgNCs in the presence of Zr(IV) ions due to electrostatic interactions between Zr(IV) and the phosphate groups, thereby capping the AgNCs. The silver enhancement process (leading to a color change to black) does not work if the AgNCs are capped. The degree of inhibition is proportional to the activity of the kinase. The color change can be detected visually or photographically in a microplate format by exploiting the changes in the grey values of the digital photos. In addition, the DNA-AgNCs display fluorescence emission at 635 nm when excited at 565 nm. Electrochemical assays were performed (at a working voltage as low as 38 mV vs. Ag/AgCl) by using a glassy carbon electrode modified with a solution containing AgNCs, Zr(IV) ions and the peptide, and immersing it into the silver enhancement solution. The assay is highly sensitive and selective. It was applied to the determination of PKA in lysates of HeLa cells. The detection limits typically are between 32 and 37 U⋅ L-1 based on a signal-to-noise ratio of 3.

The fluorescence intensity of gold nanoclusters (AuNCs) is inversely related to the length of a peptide immobilized on its surface. This finding has been exploited to design a turn-on fluorescent method for the determination of the activity of peptidase. The β-site amyloid precursor protein-cleaving enzyme 1 (BACE1) was chosen as a model peptidase. BACE1 cleaves the peptide substrates on AuNCs, and the fluorescence intensity of the AuNCs (at exCitation/emission wavelengths of 320/405 nm) carrying the rest of the cleaved peptide is significantly higher than that of the AuNCs with uncleaved peptide. Transmission electron microscopy revealed a decrease in the size of the AuNCs which is assumed cause fluorescence enhancement. The assay was applied to the determination of BACE1 activity in spiked cell lysates, and recoveries were between 96.9 and 104.0 %.

Copper nanoclusters (Cu-NCs) were prepared by reducing CuCl2 with ascorbic acid in the presence of the short peptide template Cys-Cys-Cys-Asp-Leu. They were characterized by UV-vis absorption and fluorescence spectroscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The Cu-NCs have a size of ~2 nm, can be well dispersed in water and are photostable. Their fluorescence (peaking at 425 nm under 365-nm excitation) is quenched by Fe(III) ions. Based on this finding, a sensitive and selective fluorescence assay for the detection of Fe(III) was developed. Under optimized conditions and a pH value of 2.0, the assay displays a linear response in the 0.05 to 30 μM Fe(III) concentration range, with a detection limit of 20 nM based on an S/N ratio of 3. The assay was successfully applied to the determination of Fe(III) in spiked human serum where is gave recoveries that ranged from 96.2 % to 98.3 %.

Proteins were proved to be type-independent templates for the biomineralization of iron ions into hematite mesocrystals with tunable structures and morphologies under hydrothemal conditions. Our finding could pave the way for the synthesis of mesocrystals with controlled stuctures and morphologies using templates of low-cost proteins.

A simple and sensitive fluorescence method for monitoring the activity and inhibition of protein kinase (PKA) has been developed using polycytosine oligonucleotide (dC12)-templated silver nanoclusters (Ag NCs). Adenosine-5′-triphosphate (ATP) was found to enhance the fluorescence of Ag NCs, while the hydrolysis of ATP to adenosine diphosphate (ADP) by PKA decreased the fluorescence of Ag NCs. Compared to the existing methods for kinase activity assay, the developed method does not involve phosphorylation of the substrate peptides, which significantly simplifies the detection procedures. The method exhibits high sensitivity, good selectivity, and wide linear range toward PKA detection. The inhibition effect of kinase inhibitor H-89 on the activity of PKA was also studied. The sensing protocol was also applied to the assay of drug-stimulated activation of PKA in HeLa cell lysates.

•We reported competitive electrochemical immunosensor for histamine.•We conjugated histamine with horseradish peroxidase.•Horseradish peroxidase initiated deposit of insulating film onto the electrode.•The insulating film resulted in decrease of response current of the immunosensor.•The current decrease was proportional to the concentration of histamine detected.Competitive electrochemical immunosensor for the detection of histamine based on graphene modified electrode and horseradish peroxidase (HRP) initiated deposition of insulating film was developed. After the immobilization of anti-histamine antibodies onto graphene surface, free histamine and HRP tagged histamine molecules will compete to bind with the antibodies. The captured HRP could then catalyze the polymerization of 3,3′-dimethoxybenzidine (DB) in the presence of H2O2 to produce the deposition of an insulating polymer film, poly(3,3′-dimethoxybenzidine) (PDB) onto electrode surface. The deposited insulating PDB film resulted in the decrease of the electrochemical current of the electrode in Fe(CN)63−/4− solution, and the current change is proportional to the concentration of histamine detected. The proposed immunosensor displays wide linear range towards histamine detection (1 pg/mL–1 ng/mL) with a detection limit of 0.5 pg/mL. The immunosensor was applied to the detection of histamine in serum samples with satisfactory results.

An ultrasensitive aptasensor for the detection of Mucin 1 (MUC1) protein based on fluorescence resonance energy transfer (FRET) between carbon dots (CDs) and graphene oxide (GO) is reported herein. Taking advantage of the strong fluorescence and good biocompatibility of CDs, the MUC1 aptamer was covalently conjugated to CDs (aptamer–CDs) to capture MUC1 protein through high affinity interaction between the aptamer and MUC1 protein. The FRET process between the aptamer–CDs and GO is easily achieved due to their efficient self-assembly through specific π–π interaction, in which the fluorescence of CDs was efficiently quenched. In the presence of the target MUC1 protein, the association constant between aptamer–CDs and MUC1 is bigger than that between aptamer–CDs and GO, leading to the release of the aptamer–CDs from GO, resulting in the recovery of the fluorescence of CDs. This aptamer was observed to detect MUC1 protein specifically and sensitively in a linear range from 20.0 to 804.0 nM with a detection limit of 17.1 nM. The developed aptasensor is highly biocompatible and nontoxic, which can be easily modified for the detection of other protein biomarker.

A redox hydrogel was prepared from the ferrocene (Fc) modified amino acid, phenylalanine (Phe, F), and utilized to construct an immunosensing platform for the label-free detection of a cancer biomarker prostate specific antigen (PSA). The synthesized hydrogel contains a large number of Fc moieties, which imparts the hydrogel with redox properties. The hydrogel modified electrode displays a pair of reversible redox peaks, indicating the facile electron transfer of Fc in the hydrogel at the electrode surface. After the immobilization of the anti-PSA antibody onto the hydrogel modified electrode surface and the capture of PSA molecules, the redox current at the electrode was suppressed significantly due to the blocking of the electron transfer at the electrode surface by the formed immuno-complex. The current change was proportional to the concentration of PSA detected in the range from 1 pg mL−1 to 10 ng mL−1 with a detection limit of 0.5 pg mL−1. The proposed immunosensor is simple with a low cost and high sensitivity, which could find wide clinical applications.

To increase the loading of glucose oxidase (GOx) and simplify glucose biosensor fabrication, hydrogel prepared from ferrocene (Fc) modified amino acid phenylalanine (Phe, F) was utilized for the incorporation of GOx. The synthesized hydrogel displays good biocompatibility and contains a significant number of Fc moieties, which can be considered as an ideal matrix to immobilize enzymes for the preparation of mediator-based biosensors. The hydrogel was studied by scanning electron microscopy, which indicated that it was composed of nanofibers with a diameter of around 50–100 nm and length extended to 1 mm. With the addition of GOx into the hydrogel and by directly dropping the resulting biocomposite onto the electrode surface, a glucose biosensor, that displays good performance due to improved enzyme loading and efficient electron transfer, can be simply constructed. The favorable network structure and good biocompatibility of the hydrogel could effectively avoid enzyme leakage and maintain the bioactivity of the enzymes, which resulted in good stability of the biosensor. The biosensor was utilized for the detection of glucose in blood samples with results comparable to those obtained from the hospital. The hydrogel as a functional component of an amperometric biosensor has implications for future development of biosensors and for clinical applications.

Ferrocene (Fc)-tagged peptide nanowires (Fc-PNWs) were synthesized via the self-assembly of Fc-coupled diphenylalanine (Phe-Phe, FF) and then used as supporting matrix for immobilization of glucose oxidase (GOx). Scanning electron microscopy (SEM) characterization indicated that the Fc-PNWs were twisted together with the diameter around 50 nm. The GOx-functionalized Fc-PNWs contained both mediator Fc and GOx for the electrochemical detection of glucose. Thus, by simply dropping the biocomposite onto the electrode surface in a single step, the resulting biosensor displays high sensitivity, wide linear range and good stability towards glucose detection. The good performance of the biosensor originated from the large amount of Fc moieties contained in the nanowire and the facile electron transfer between Fc and GOx. For real sample analysis, the glucose contents in blood samples determined by the biosensor were in good agreement with those obtained using the glucose detection kit. The simplicity of the biosensor preparation process enables mass production of the biosensor with broad potential commercial applications. The synthesized Fc-PNWs can also be used in diverse sensing and biosensing fields.

•Simple and low-cost paper based colorimetric biosensing platform was developed.•Cross-linked 3-aminopropyltriethoxysilane was utilized as colorimetric probe.•H2O2, glucose and protein biomarker can be detected by the paper based biosensor.•The detection was based on H2O2 induced color change of the probe.•The detection can be achieved through naked eyes and by image software.Paper based colorimetric biosensing platform utilizing cross-linked siloxane 3-aminopropyltriethoxysilane (APTMS) as probe was developed for the detection of a broad range of targets including H2O2, glucose and protein biomarker. APTMS was extensively used for the modification of filter papers to develop paper based analytical devices. We discovered when APTMS was cross-linked with glutaraldehyde (GA), the resulting complex (APTMS–GA) displays brick-red color, and a visual color change was observed when the complex reacted with H2O2. By integrating the APTMS–GA complex with filter paper, the modified paper enables quantitative detection of H2O2 through the monitoring of the color intensity change of the paper via software Image J. Then, with the immobilization of glucose oxidase (GOx) onto the modified paper, glucose can be detected through the detection of enzymatically generated H2O2. For protein biomarker prostate specific antigen (PSA) assay, we immobilized capture, not captured anti-PSA antibody (Ab1) onto the paper surface and using GOx modified gold nanorod (GNR) as detection anti-PSA antibody (Ab2) label. The detection of PSA was also achieved via the liberated H2O2 when the GOx label reacted with glucose. The results demonstrated the possibility of this paper based sensor for the detection of different analytes with wide linear range. The low cost and simplicity of this paper based sensor could be developed for “point-of-care” analysis and find wide application in different areas.

Sensitive electrochemical immunosensor for the detection of interleukin-17 (IL-17) was prepared using cadmium (Cd2+) ion incorporated polystyrene sphere (PS, PS–Cd2+) as label. With the linking of the detection anti-IL-17 antibody (Ab2) onto PS (PS–Cd2+–Ab2), the modified PS can be captured onto the electrode surface through immuno-reaction. Tetrahydrofuran (THF) was then dropped onto the electrode surface to dissolve the PS and release the contained Cd2+ ions onto electrode surface for the following stripping voltammetric detection. Compared to the electrochemical label based on quantum dots (QD), this method for the detection of Cd2+ ion is much simpler and straightforward. With the immobilization of primary anti-IL-17 antibody (Ab1) onto graphene surface, the resulting immunosensor displays high sensitivity, which can be ascribed to the significant number of Cd2+ ions incorporated into PS and a large number of Ab2 linked onto PS surface. The response current was linear to the logarithm of IL-17 concentration in the range from 0.1 pg to 1 ng/mL with a detection limit of 50 fg/mL. The immunosensor results were validated through the detection of IL-17 in serum samples with satisfactory results.

A novel kind of hydrogel was prepared from ferrocene (Fc) functionalized amino acid phenylalanine (Phe) and applied for the fabrication of electrochemical immunosensor to detect tumor necrosis factor α (TNF-α). The prepared hydrogel displays high redox activity due to the incorporation of a significant number of Fc moieties into the hydrogel. When the hydrogel was reacted with ascorbic acid (AA), the redox activity of the hydrogel was decreased as the Fc moieties in the hydrogel were reduced by AA. Based on this phenomenon, the immunosensor was constructed utilizing the traditional sandwich format using the hydrogel modified electrode as sensing platform and alkaline phosphatase (ALP) modified polystyrene sphere (PS) as label. First, primary anti-TNF-α antibody was immobilized onto the electrode surface via gold nanoparticles (AuNPs), then TNF-α and ALP were sequentially captured onto electrode. The ALP captured onto the electrode will catalyzes the hydrolysis of ascorbic acid 2-phosphate (AA-p) to form AA. The latter, in return, will cause the decrease of the redox current of the electrode. The decrease of the redox current is proportional to the concentration of TNF-α detected in the range from 1 pg/mL to 10 ng/mL. The immunosensor testing results were validated through the detection of clinical serum sample with satisfactory results.

•Ferrocene tagged peptide nanowire was synthesized.•The nanowire was made water soluble and utilized as label for immunosensor.•Each nanowire contains approximately 5×105 of ferrocene moieties.•Significantly enhanced sensitivity was obtained for the detection of human IgG.A ferrocence (Fc)-tagged peptide comprising the phenylalanine–phenylalanine (Phe–Phe) sequence was synthesized and allowed to self-assemble into uniform nanowires with a diameter of ca. 100 nm and lengths in the range of 5–10 µm. The Fc-tagged peptide nanowire (Fc-PNW) become well dispersed in aqueous solution when coated with poly(diallyldimethylammonium chloride) (PDDA). Gold nanoparticles (AuNPs) and antibody molecules can then be adsorbed onto the Fc-PNW surface. The resultant antibody modified Fc-PNW was explored as a detection probe for sensitive electrochemical immunosensing in a sandwich assay wherein the capture antibody was attached onto a graphene/gold nanoparticle (AuNPs–GN) composite film. The as prepared electrochemical immunosensor possesses a low detection limit (5 fg/mL) for human IgG and a wide linear range encompassing four orders of magnitude (from 10 fg/mL to 100 pg/mL). Such a low detection limit stems from the significant signal amplification by the large number of Fc moieties on the PNW (for a 5-µm-long Fc-PNW, the number of Fc moieties is 5×105). The electrochemical immunosensor is also highly selective and the sensor was demonstrated to be amenable to real sample analysis.

Sensitive electrochemical immunosensor for the detection of protein biomarker tumor necrosis factor α (TNF-α) was reported that uses ferrocene carboxylic acid (Fc) functionalized self-assembled peptide nanowire (Fc-PNW) as sensor platform and glucose oxidase (GOx) modified gold nanorod (GNR) as label. Greatly enhanced sensitivity is achieved based on a dual signal amplification strategy: first, the synthesized Fc-PNW used as the sensor platform increased the loading of primary anti-TNF-α antibody (Ab1) onto electrode surface due to its large surface area. At the same time, the Fc moiety on the nanowire is used as a mediator for GOx to catalyze the glucose reaction. Second, multiple GOx and secondary anti-TNF-α antibody (Ab2) molecules are bounded onto each GNR to increase the sensitivity of the immunosensor. After the preparation of the immunosensor based on the traditional sandwich protocol, the response of the immunosensor towards glucose was used as a signal to differentiate various concentrations of TNF-α. The resulting immunosensor has high sensitivity, wide linear range (0.005–10 ng/mL) and good selectivity. This immunosensor preparation strategy is a promising platform for clinical screening of protein biomarkers.Highlights► Ferrocene functionalized self-assembled peptide nanowire was synthesized. ► Electrochemical immunosensor was prepared based on the nanowire and gold nanorod. ► Sensitive detection of tumor necrosis factor α was achieved.

•Supramolecular hydrogel was prepared from ferrocene modified amino acid.•The hydrogel modified electrode was utilized as sensing platform for immunosensor.•The reaction of the hydrogel with H2O2 caused the decrease of the redox current.•Glucose oxidase was used as detection antibody label.•The resulting immunosensor displays high sensitivity and good selectivity.A simple and novel supramolecular hydrogel was prepared from ferrocene (Fc) modified amino acid phenylalanine (F) and utilized as electrochemical immunosensing platform for the detection of human IgG. Scanning electron microscopy (SEM) characterization indicated that the hydrogel is composed of fibrils with diameter around 50–100 nm and length extend to 1 mm. When the prepared hydrogel was reacted with H2O2, the Fc moieties on the amino acid was oxidized, leading to the disruption of the hydrogel structure and the decrease of its redox signal, which was characterized in detail by SEM and the electrochemical method. Regarding the redox current decrease upon the reaction of the hydrogel with H2O2, the hydrogel modified electrode was utilized as immunosensing interface. After the construction of the immunosensor based on the traditional sandwich protocol with glucose oxidase (GOx) functionalized carbon nanotube (CNT) as detection antibody label, the GOx attached onto electrode surface would catalyze glucose reaction to produce H2O2 and cause the decrease of redox current of the electrode. The current change is proportional to the concentration of IgG detected in the range from 0.1 to 100 pg/mL. The high sensitivity, wide linear range and good reproducibility of the immunosensor indicate this immunosensing platform can be easily extended to the detection of other protein biomarkers.

A novel catalyst of Pt/Fe3O4–CeO2 is prepared by anchoring Pt nanoparticles onto Fe3O4 coated cerium (CeO2) surface. The catalyst is characterized by TEM and tested for methanol electrooxidation in acid solution. The results demonstrated enhanced catalytic performance of Pt/Fe3O4–CeO2 toward methanol oxidation. The catalytic current of Pt/Fe3O4–CeO2 towards methanol is about 4 times higher than that of Pt–CeO2, indicating the introduction of Fe3O4 significantly improved the catalytic activity of the catalyst. The incaresed catalytic activity can be ascribed to the increased conductivity of the catalyst.Highlights► A novel catalyst of Pt/Fe3O4–CeO2 was prepared. ► The catalyst was tested for methanol electrooxidation in acid solution. ► The results shows enhanced performance of Pt/Fe3O4–CeO2 for methanol oxidation.

Palladium modified cerium oxide (CeO2 or ceria) nanoparticles (Pd-CeO2) were prepared by depositing palladium nanoparticles with average diameters of 3–5 nm on the surface of CeO2 via chemical reduction of the precursor (Pd2+). The electrocatalytic activity of Pd-CeO2 toward the detection of different important compounds (i.e. glucose, ascorbic acid (AA), dopamine (DA) and uric acid (UA)) has been investigated. Compared with the CeO2 or Pd modified electrode, the Pd-CeO2 modified electrode has higher catalytic activity toward the oxidation of glucose, AA, DA, and UA. A nonenzymatic glucose sensor was developed for the sensitive and selective detection of glucose. Moreover, in the mixture of AA, DA and UA, obvious electrochemical potential separations among the three detected peaks were observed, making the simultaneously determination of AA, DA and UA possible. The attractive features of Pd-CeO2 provide potential applications in sensor and biosensor design.

In this paper, thionine was electro-polymerized onto the surface of carbon nanotube (CNT)-modified glassy carbon (GC) to fabricate the polythionine (PTH)/CNT/GC electrode. It was found that the electro-reduction current of nitrite was enhanced greatly at the PTH/CNT/GC electrode. It may be demonstrated that PTH was used as a mediator for electrocatalytic reduction of nitrite, and CNTs as an excellent nanomaterial can improve the electron transfer between the electrode and nitrite. Therefore, based on the synergic effect of PTH and CNTs, the PTH/CNT/GC electrode was employed to detect nitrite, and the high sensitivity of 5.81 μA mM− 1, and the detection limit of 1.4 × 10− 6 M were obtained. Besides, the modified electrode showed an inherent stability, fast response time, and good anti-interference ability. These suggested that the PTH/CNT/GC electrode was favorable and reliable for the detection of nitrite.Highlights► Polythionine (PTH) was used as a mediator for electrocatalytic reduction of nitrite. ► Carbon nanotubes (CNTs) improve electron transfer between the electrode and nitrite. ► The PTH/CNT/glassy carbon electrode showed excellent nitrite detection performance.

Ferrocene (Fc)-tagged peptide nanowires (Fc-PNWs) were synthesized via the self-assembly of Fc-coupled diphenylalanine (Phe-Phe, FF) and then used as supporting matrix for immobilization of glucose oxidase (GOx). Scanning electron microscopy (SEM) characterization indicated that the Fc-PNWs were twisted together with the diameter around 50 nm. The GOx-functionalized Fc-PNWs contained both mediator Fc and GOx for the electrochemical detection of glucose. Thus, by simply dropping the biocomposite onto the electrode surface in a single step, the resulting biosensor displays high sensitivity, wide linear range and good stability towards glucose detection. The good performance of the biosensor originated from the large amount of Fc moieties contained in the nanowire and the facile electron transfer between Fc and GOx. For real sample analysis, the glucose contents in blood samples determined by the biosensor were in good agreement with those obtained using the glucose detection kit. The simplicity of the biosensor preparation process enables mass production of the biosensor with broad potential commercial applications. The synthesized Fc-PNWs can also be used in diverse sensing and biosensing fields.

Proteins were proved to be type-independent templates for the biomineralization of iron ions into hematite mesocrystals with tunable structures and morphologies under hydrothemal conditions. Our finding could pave the way for the synthesis of mesocrystals with controlled stuctures and morphologies using templates of low-cost proteins.

An ultrasensitive aptasensor for the detection of Mucin 1 (MUC1) protein based on fluorescence resonance energy transfer (FRET) between carbon dots (CDs) and graphene oxide (GO) is reported herein. Taking advantage of the strong fluorescence and good biocompatibility of CDs, the MUC1 aptamer was covalently conjugated to CDs (aptamer–CDs) to capture MUC1 protein through high affinity interaction between the aptamer and MUC1 protein. The FRET process between the aptamer–CDs and GO is easily achieved due to their efficient self-assembly through specific π–π interaction, in which the fluorescence of CDs was efficiently quenched. In the presence of the target MUC1 protein, the association constant between aptamer–CDs and MUC1 is bigger than that between aptamer–CDs and GO, leading to the release of the aptamer–CDs from GO, resulting in the recovery of the fluorescence of CDs. This aptamer was observed to detect MUC1 protein specifically and sensitively in a linear range from 20.0 to 804.0 nM with a detection limit of 17.1 nM. The developed aptasensor is highly biocompatible and nontoxic, which can be easily modified for the detection of other protein biomarker.