Faraday cage-type self-powered immunosensor based on hybrid enzymatic biofuel cell.

Self-powered immunosensors (SPIs) based on enzymatic biofuel cell (EBFC) have low sensitivity and poor stability due to the high impedance of the immune sandwich and the vulnerability of enzymes to environmental factors. Here, we applied the Faraday cage-type sensing mode on a hybrid biofuel cell (HBFC)-based SPI for the first time, which exhibited high sensitivity and stability. Cytokeratin 19 fragment (CYFRA 21-1) was used as a model analyte. Au nanoparticle-reduced graphene oxide (Au-rGO) composite was used as the supporting matrix for immunoprobe immobilized with detection antibody and glucose dehydrogenase (GDH), also the builder for Faraday cage structure on the bioanode in the presence of antigen. After the combination of immunoprobe, antigen, and the antibody on the bioanode, the Faraday cage was constructed in case the AuNP-rGO was applied as a conductive cage for electron transfer from GDH to the bioanode without passing through the poorly conductive protein. With the assistance of the Faraday cage structure, the impedance of the bioanode decreased significantly from 4000 to 300 Ω, representing a decline of over 90%. The sensitivity of the SPI, defined as the changes of open circuit voltage (OCV) per unit concentration of the CYFRA 21-1, was 68 mV [log (ng mL-1)]-1. In addition, Fe-N-C was used as an inorganic cathode material to replace enzyme for oxygen reduction reaction (ORR), which endowed the sensor with 4-week long-term stability. This work demonstrates a novel sensing platform with high sensitivity and stability, bringing the concept of hybrid biofuel cell-based self-powered sensor.

Anti-Fouling Strategies of Electrochemical Sensors for Tumor Markers.

The early detection and prognosis of cancers require sensitive and accurate detection methods; with developments in medicine, electrochemical biosensors have been developed that can meet these clinical needs. However, the composition of biological samples represented by serum is complex; when substances undergo non-specific adsorption to an electrode and cause fouling, the sensitivity and accuracy of the electrochemical sensor are affected. In order to reduce the effects of fouling on electrochemical sensors, a variety of anti-fouling materials and methods have been developed, and enormous progress has been made over the past few decades. Herein, the recent advances in anti-fouling materials and strategies for using electrochemical sensors for tumor markers are reviewed; we focus on new anti-fouling methods that separate the immunorecognition and signal readout platforms.

Universal and High-Speed Zeptomolar Protein Serum Assay with Unprecedented Sensitivity.

The accurate detection of trace protein biomarkers is critical for disease diagnosis, healthcare, and pathology research. Currently, the main predicaments of techniques are low sensitivity, prolonged procedures, and the need for specialized devices. Moreover, multistep handling and nonspecific biofouling can lead to high background noise and false positives. To overcome these barriers, a novel ultrasensitive electrochemical platform was developed by combining an electrochemistry approach with the silver mirror reaction to detect proteins at the zeptomolar level. This assay can be accomplished in about only 18 min. As a proof of the concept, human immunoglobulin G (h-IgG) as a model analyte exhibited an ultralow detection limit of 6.31 ag mL-1 (0.04 zeptomoles mL-1). This strategy can be exploited as a universal approach for the ultrasensitive detection of various proteins in clinical diagnostics and point-of-care testing.

Electrochemical Signal Substance for Multiplexed Immunosensing Interface Construction: A Mini Review.

Appropriate labeling method of signal substance is necessary for the construction of multiplexed electrochemical immunosensing interface to enhance the specificity for the diagnosis of cancer. So far, various electrochemical substances, including organic molecules, metal ions, metal nanoparticles, Prussian blue, and other methods for an electrochemical signal generation have been successfully applied in multiplexed biosensor designing. However, few works have been reported on the summary of electrochemical signal substance applied in constructing multiplexed immunosensing interface. Herein, according to the classification of labeled electrochemical signal substance, this review has summarized the recent state-of-art development for the designing of electrochemical immunosensing interface for simultaneous detection of multiple tumor markers. After that, the conclusion and prospects for future applications of electrochemical signal substances in multiplexed immunosensors are also discussed. The current review can provide a comprehensive summary of signal substance selection for workers researched in electrochemical sensors, and further, make contributions for the designing of multiplexed electrochemical immunosensing interface with well signal.

In situ growth of electroactive polymers via ATRP to construct a biosensing interface for tumor marker.

A novel biosensing interface for tumor markers was designed based on the atom transfer radical polymerization (ATRP) of poly(isopropenylphenol) (PPPL) in situ initiated by the fixing of p-chloromethyl benzoic acid on the surface of amino-modified electrodes. It was found that the electrochemical activity of PPPL itself can provide sufficient signals for these biosensors, which can avoid signal leakage and streamline the interface modification process. Cu(II) ions absorbed on the carbon spheres and then were released via acid stimulation to act as a catalyst to participate in the interface polymerization with ATRP. As the concentration of targets increased, more Cu(II) ions were released, and the electrochemical signal of polymers was enhanced. Therefore, the sensitive detection of carbohydrate antigen 19-9 (CA19-9) as a model target was achieved, with an ultralow limit of detection of 39 µU mL-1 and wide detection range from 100 µU mL-1 to 100 U mL-1 under optimal conditions. Furthermore, this method achieved satisfying performance in human blood serum with good inter-assay precision (RSD < 6%) and satisfactory recovery of ~ 99-105%. According to the results, this work is of great significance for constructing biosensor interfaces via in situ polymerization. A novel biosensing interface for tumor marker was designed based on atom transfer radical polymerization (ATRP), which poly(isopropenylphenol) with electrochemical signal was fabricated in situ on electrode.

Ultrasensitive amperometric immunosensor for the prostate specific antigen by exploiting a Fenton reaction induced by a metal-organic framework nanocomposite of type Au/Fe-MOF with peroxidase mimicking activity.

To increase the sensitivity of electrochemical sensor, Fe-MIL-88B-NH2 (Fe-MOF) with peroxidase-like activity is designed for the construction of immunoprobe. The Fe-MOF was prepared by one-step hydrothermalf method using 2-aminoterephthalic acid and iron(III) chloride. For the immunoprobe, it was fabricated by gold nanocomposite/Fe-MOF (Au/Fe-MOF) for the immobilization of labeling antibody (the antibody was used to conjuncting with label materials). The thin layer of Methylene Blue (MB) covered by reduced graphene oxide-gold nanocomposites (Au-rGO) serves as a substrate to covalently fix coating antibodies. The MB as a redox-active species was modified on the glass carbon electrode that can give a strong amperometric signal at 0.18 V (vs. Ag/AgCl). With the participation of H2O2, Fe-MOF can induce the Fenton reaction which degrades MB covered by Au-rGO on the substrate. The rest of MB on the surface of electrode becomes oxidized thereby generating a current signal. Square wave voltammetry (SWV) was used to quantify PSA. Under optimal conditions, the immunoassay is stable, specific and reproducible. It has a lower detection limit of 0.13 pg mL-1 (S/N = 3) and a wide analytical range that extends from 0.001 to 100 ng mL-1. Graphical abstractA sandwich-type amperometric immunoassay based on Fe-MOF-induced Fenton reaction was designed for sensitive determination of prostate specific antigen.

Label-free electrochemical immunosensor for ultrasensitive detection of neuron-specific enolase based on enzyme-free catalytic amplification.

Enzyme-free catalytic amplification is of great significance for sensitive label-free electrochemical immunosensors. In this study, an enzyme-free catalytic amplification based label-free amperometric immunosensor was developed for sensitive detection of neuron-specific enolase (NSE) by use of a AuPd nanoparticle-multiwalled carbon nanotube (AuPd-MWCNT) composite, ferrocenecarboxaldehyde (Fc-CHO), and chitosan hybrid hydrogel. The intrinsic virtues of chitosan not only resulted in bioactivity of the attached antibodies and made the other component of the immunosensor easier to fix on the electrode, but also imparted abundant binding sites to the hydrogel to condense Fc-CHO to achieve the initial signal amplification. Fc-CHO, which served as an electroactive species to generate a redox response, also exhibits excellent electrocatalytic activity toward H2O2. AuPd-MWCNT composite, with enhanced peroxidase-like catalytic activity, could catalyze H2O2 to accelerate electron transfer. When H2O2 was present in the detection solution, synergetic catalysis of Fc-CHO and AuPd-MWCNT composite toward H2O2 was achieved, thus realizing enzyme-free signal amplification. On the basis of this enzyme-free signal amplification, the electrochemical immunosensing platform provided a wide linear range from 1 pg mL-1 to 100 ng mL-1, a low detection limit of 0.483 pg mL-1, and high sensitivity of 7.22 µA (log10 C NSE)-1. Moreover, the immunosensor showed enormous potential in clinical application. Graphical abstract An enzyme-free catalytic amplification based label-free amperometric immunosensor was developed for sensitive detection of neuron-specific enolase (NSE) by use of a AuPd nanoparticle-multiwalled carbon nanotube (MWCNT) composite, ferrocenecarboxaldehyde (Fc-CHO), and chitosan (CS) hybrid hydrogel. BSA bovine serum albumin, GA glutaraldehyde, SWV square wave voltammetry.

Sensitive electrochemical detection of copper ions based on the copper(II) ion assisted etching of Au@Ag nanoparticles.

A new sensitive electrochemical sensor for the detection of copper ions based on the copper ion assisted etching of Au@Ag nanoparticles was developed in this work. Since copper ions could greatly catalyze the etching process of the silver shell of Au@Ag nanoparticles in the presence of thiosulfate solutions, leading to an obvious decrease of the linear sweep voltammetry (LSV) signals of silver, the concentration of the copper ions, therefore, can be measured. Under the optimized conditions, the electrochemical sensor exhibited excellent sensitivity and selectivity for Cu(2+), with wide linear ranges of 1 nM to 100 µM, and the detection limit of 0.3 nM. In addition, this method was successfully applied for the analysis of Cu(2+) in river water and exhibited good analytical performance.

A simple and novel system for colorimetric detection of cobalt ions.

A simple and novel method for the colorimetric detection of Co(2+) was developed based on controlling the oxidation level of methylene blue (MB). After a complex was formed between MB, 2-aminothiophenol (ATP) and copper nitrate (MB-ATP-Cu(2+)), the sensing of Co(2+) showed high selectivity. The mechanism of sensing has also been discussed.

Novel dual-responsive hydrogel composed of polyacrylamide/Fe-MOF/zinc finger peptide for construction of electrochemical sensing platform.

Responsive hydrogels have received much attention for improving the detection performance of electrochemical sensors because of their special responsiveness. However, current responsive hydrogels generally suffer from long response times, ranging from tens of minutes to several hours. This situation severely limits the detection performance and practical application of electrochemical sensors. Here, an electrochemical sensing platform was constructed by employing dual-responsive polyacrylamide/zinc finger peptide/Fe-MOF hydrogel (PZFH) as the silent layer, sodium alginate-Ni2+-graphene oxide hydrogel as the signal layer. GOx@ZIF-8, as the immunoprobe, catalyzed glucose to H2O2 and gluconic acid, resulting in the cleavage of immunoprobe as the pH decreased and subsequent release of Zn2+ ions. During the process of Fe-MOF converting from Fe3+ to Fe2+, free radicals were generated and used to destroy the structure of the PZFH. Cysteine and histidine in the zinc finger peptide can specifically bind to Zn2+ to create many pores in PZFH, exposing the signal layer. These synergistic effects rapidly decreased the impedance of PZFH and increased the electrochemical signal of Ni2+. The electrochemical sensing platform was used to detect pro-gastrin-releasing peptide with response times as short as 7 min of PZFH, a wide linear range from 100 ng mL-1 to 100 fg mL-1, and an ultra-low limit of detection of 14.24 fg mL-1 (S/N = 3). This strategy will provide a paradigm for designing electrochemical sensors.

Quantification of Glycated Hemoglobin in Total Hemoglobin by a Simultaneous Dual-Signal Acquisition Approach.

The glycated hemoglobin (HbA1c) level, which is defined as the ratio of HbA1c to total hemoglobin (tHb, including glycated and unglycated hemoglobin), is considered one of the preferred indicators for diabetes monitoring. Generally, assessment of the HbA1c level requires separate determination of tHb and HbA1c concentrations after a complex separation step. This undoubtedly increases the cost of the assay, and the loss or degradation of HbA1c during the separation process results in a decrease in the accuracy of the assay. Therefore, this study explored a dual-signal acquisition method for the one-step simultaneous evaluation of tHb and HbA1c. Quantification of tHb: graphene adsorbed carbon quantum dots and methylene blue were utilized as the substrate material and linked to the antibody. tHb was captured on the substrate by the antibody. The unique heme group on tHb catalyzed the production of •OH from H2O2 to degrade methylene blue on the substrate, and a quantitative relationship between the tHb concentration and the methylene blue oxidation current signal was constructed. Quantification of HbA1c: complex labels with HbA1c recognition were made of ZIF-8-ferrocene-gold nanoparticles-mercaptophenylboronic acid. The specific recognition of the boronic acid bond with the unique cis-diol structure of HbA1c establishes a quantitative relationship between the oxidation current of the label-loaded ferrocene and the concentration of HbA1c. Thus, the HbA1c level can be assessed with only one signal readout. The sensor exhibited extensive detection ranges (0.200-600 ng/mL for tHb and 0.100-300 ng/mL for HbA1c) and low detection limits (4.00 × 10-3 ng/mL for tHb and 1.03 × 10-2 ng/mL for HbA1c).

A facile electrochemical immunosensor based on EDTA-Pb2+ complexation reaction.

The sensitivity of electrochemical (EC) sensors has been improved through the development of multiple approaches. However, the majority of EC sensors were limited in their practical application by high costs or tedious procedures. Herein, based on ethylenediaminetetraacetic acid (EDTA)-Pb2+ complexation reaction, a facile and affordable immunosensor was designed. Pb2+-magnesium silicate hydrate was served as the sensing substrate. The immunorecognition process was carried out in the Eppendorf tube, and antibody-functionalized Pb2+-polydopamine was utilized as immunoprobe. In the tube, the quantitative and appropriate excess of EDTA was introduced to complex with Pb2+ on the immunoprobes. The remaining EDTA was added to the sensing substrate surface to coordinate with some Pb2+ in it. This leaded to the reduction of the EC signal of Pb2+, which was related to the antigen concentration. Using prostate-specific antigen as the model analyte, the sensitive detection was realized with a low limit of detection (30.49 fg mL-1). Remarkably, the assay results were available within 24 min, sensibly faster than the most existing EC sensors.

Customized Enhancement of Thermal Sensitivity of Tumors at Different Subcutaneous Depths by Multichannel Lanthanide Nanocomposites.

The photothermal therapeutic effect on tumors located at different subcutaneous depths varies due to the attenuation of light by tissue. Here, based on the wavelength-dependent optical attenuation properties of tissues, the tumor depth is assessed using a multichannel lanthanide nanocomposite. A zeolitic imidazolate framework (ZIF-8)-coated nanocomposite is able to deliver high amounts of the hydrophilic heat shock protein 90 inhibitor epigallocatechin gallate through a hydrogen-bonding network formed by the encapsulated highly polarized polyoxometalate guest. It is superior to both bare and PEGylated ZIF-8 for drug delivery. With the assessment of tumor depth and accumulated amount of nanocomposite by fluorescence, an irradiation prescription can be customized to release sufficient HSP90 inhibitor and generate heat for sensitized photothermal treatment of tumors, which not only ensured therapeutic efficacy but also minimized damage to the surrounding tissues.

A new luminol chemiluminescence sensor for glucose based on pH-dependent graphene oxide.

In this study, graphene oxide (GO) was found to catalyze the luminol-O2 reaction, which yielded a novel chemiluminescence (CL). Remarkably, the CL emission could be tuned by modulating the pH of the GO dispersion. Transmission electron microscopy, CL spectra, electron spin resonance spectra studies were carried out to investigate the CL mechanism. The results indicate that the CL emission was attributed to the intrinsic catalytic effect of GO acting as the radical generation proliferators and electron transfer accelerators. Based on the GO catalyzed luminol-O2 system, we successfully developed a new CL sensor to detect glucose. Under the optimized conditions, glucose could be assayed in the range of 0.05 mM to 5 mM with a detection limit of 0.044 mM. For the detection of clinical serum samples, it is well consistent with the data determined by commercially available method in hospital, indicating that the new CL method provides a possible application for the detection of glucose in clinical diagnostics.

Facile fabrication of truncated octahedral Au nanoparticles and its application for ultrasensitive surface enhanced Raman scattering immunosensing.

Monodispersed truncated octahedral (TOH) Au nanoparticles (NPs) with an average edge-length of about 16 nm were synthesized using poly(diallyldimethylammonium chloride) (PDDA) both as a stabilizing and reducing agent via a one-step reaction. Remarkably, no seeds, surfactants or additional reductant were used in this reaction. In addition, the PDDA molecules on the surface of the TOH AuNPs make them convenient for use in layer-by-layer assembly by electrostatic interactions. Importantly, the TOH AuNPs show a significant surface enhanced Raman scattering (SERS) activity, and can be directly used for building SERS-active substrates and tags. Based on these promising properties, an ultrasensitive SERS-based immunosensing platform was developed. Using human immunoglobulin (h-IgG) as a model target analyte, a detection limit of 36.56 fg ml(-1) was reached.

Electrochemical sensing interface based on the oriented self-assembly of histidine labeled peptides induced by Ni2+ for protease detection.

To construct an electrochemical sensing interface which was convenient for protease recognition and cleavage, we designed a strategy for directed self-assembly of histidine-tagged peptides on the electrode led by Ni2+ ions for electrochemical detection of prostate specific antigen (PSA). The electrode surface was first functionalized using carboxylated multiwalled carbon nanotubes and then modified with the metal ion chelating agent (5 S)-N-(5-Amino-1-carboxypentyl) iminodiacetic acid (NIA). After the Ni2+ was captured by NIA, the designed immune-functional peptide could be oriented assembly to the electrode interface through the imidazole ring of histidine at the tail, completing the construction of the recognition layer. Therefore, by adding the analyte PSA to identify and shear the immune-functional peptide, the ferrocene in its head was released, resulting in a reduction in the electrical signal, enabling sensitive detection. In addition, the self-assembly layer could be removed by pickling to realize the reconstruction of the recognition layer. Under optimal conditions, the electrochemical sensor had an ultralow detection limit of 11.8 fg mL-1 for PSA, with a wide detection range from 1 pg mL-1 to 100 ng mL-1. In this work, an electrochemical sensing interface based on the histidine-tagged peptide induced by Ni2+ was formed to enable controllable oriented assembly on the electrode surface, and the recognition layer could be reconstructed via pickling, providing a potential approach for the design of repeatable interfaces.

An ultrasensitive chemiluminescence biosensor for carcinoembryonic antigen based on autocatalytic enlargement of immunogold nanoprobes.

A sensitive flow injection chemiluminescence assay for carcinoembryonic antigen (CEA) detection based on signal amplification with gold nanoparticles (NPs) is reported in the present work. The sandwich system of CEA/anti-CEA/goat-anti-mouse IgG functionalized Au nanoparticles was used as the sensing platform. In order to improve detection sensitivity, a further gold enlargement step was developed based on the autocatalytic Au deposition of gold nanoprobes via the reduction of AuCl(4)- to Au0 on their surface in the presence of NH(2)OH·HCl. AuCl(4)-, which is a soluble product of gold nanoprobes, served as an analyte in the CL reaction for the indirect measurement of CEA. Under optimized conditions, the CL intensity of the system was linearly related to the logarithm of CEA concentration in the range of 100 pg∙mL-1 to 1,000 ng∙mL-1, with a detection limit of 20 pg∙mL-1.

Proton-responsive annunciator based on i-motif DNA structure modified metal organic frameworks for ameliorative construction of electrochemical immunosensing interface.

Reformative exploitation for metal organic frameworks (MOFs) has been a topic subject in electrochemical sensing, in which the loading of electroactive species is always introduced to enable them to generate electrochemical signal. However, insulation shielding of MOFs and flimsy combination method interfere with the signal readout of electroactive dyes when they are co-immobilized on electrode surface, indicating that an amelioration is imperatively proposed to solve these issues. Herein, a proton-activated annunciator for responsive release of methylene blue (MB) based on i-motif DNA structure modified UIO-66-NH2 was presented to design electrochemical immunosensor (Squamous cell carcinoma antigen was used as the model analyte). With the catalysis of a ZIF-8 immunoprobe contained glucose oxidase (GOx) to glucose in test tube, protons are produced in ambient solution and then they can be used as the key to unlock the i-motif functionalized UIO-66-NH2, releasing the loaded MB molecules to be readout on an improved electrode. This stimuli-responsive mode not merely eliminates the insulation effect of MOFs but also provides a firm loading method for electroactive dyes. Under the optimal conditions, the proposed immunoassay for SCCA had displayed excellent performance with a wide linear range from 1 µg mL-1 to 1 pg mL-1 and an ultralow detection limit of 1.504 fg mL-1 (S/N = 3) under the optimal conditions.

Synthesis of Ag(2) S-Ag nanoprisms and their use as DNA hybridization probes.

A simple synthetic route to prepare Ag(2) S-Ag nanoprisms consists of the facile addition of Na(2) S to a solution of triangular Ag nanoprisms. The resulting Ag(2) S-Ag nanoparticles are more stable in solution than the original Ag nanoprisms, and two surface plasmon resonance (SPR) bands of the original Ag nanoprisms still remain. In addition, the SPR bands of the Ag(2) S-Ag nanoprisms are tunable over a wide range. The Ag(2) S-Ag nanoprisms can be directly bioconjugated via well-established stable Ag(2) S surface chemistry with readily available sulfur coupling agents. The nanoprisms are used in the hybridization of functionalized oligonucleotides, and show promise as probes for future biosensing applications.

Sculpturing effect of sodium thiosulfate in shape transformation of silver nanoparticles from triangular nanoprisms to hexagonal nanoplates.

Sodium thiosulphate (Na2S2O3) solution was used to etch silver nanoparticles (NPs) and transform them from triangular nanoprisms to hexagonal nanoplates. UV-Vis spectroscopy and transmission electron microscopy were used to monitor the evolution of the hexagonal Ag nanoplates. S2O3(2-) etched the corners of the triangular Ag nanoprisms, and the Ag atoms that were removed aggregated into small clusters. The facet-selective etching effect of S2O3(2-) can be mainly attributed to the surface energy difference of each face of the nanoplate. The mechanism of S2O3(2-) etching also involves formation of Ag2S2O3 on the nanoprisms vertices, which is followed by its hydrolysis and subsequent dissociation of Ag atoms to form stable Ag2S. The hexagonal Ag nanoplates showed higher surface-enhanced Raman scattering activity than the original nanoprisms.