Wenju Xu

A sensitive electrochemical aptasensor based on palladium nanoparticles decorated graphene–molybdenum disulfide flower-like nanocomposites and enzymatic signal amplification

In the present study, with the aggregated advantages of graphene and molybdenum disulfide (MoS2), we prepared poly(diallyldimethylammonium chloride)-graphene/molybdenum disulfide (PDDA-G-MoS2) nanocomposites with flower-like structure, large surface area and excellent conductivity. Furthermore, an advanced sandwich-type electrochemical assay for sensitive detection of thrombin (TB) was fabricated using palladium nanoparticles decorated PDDA-G-MoS2 (PdNPs/PDDA-G-MoS2) as nanocarriers, which were functionalized by hemin/G-quadruplex, glucose oxidase (GOD), and toluidine blue (Tb) as redox probes. The signal amplification strategy was achieved as follows: Firstly, the immobilized GOD could effectively catalyze the oxidation of glucose to gluconolactone, coupling with the reduction of the dissolved oxygen to H2O2. Then, both PdNPs and hemin/G-quadruplex acting as hydrogen peroxide (HRP)-mimicking enzyme could further catalyze the reduction of H2O2, resulting in significant electrochemical signal amplification. So the proposed aptasensor showed high sensitivity with a wide dynamic linear range of 0.0001 to 40 nM and a relatively low detection limit of 0.062 pM for TB determination. The strategy showed huge potential of application in protein detection and disease diagnosis.

Ultrasensitive thrombin detection based on direct electrochemistry of highly loaded hemoglobin spheres-encapsulated platinum nanoparticles as labels and electrocatalysts.

For the first time, a sandwich-type electrochemical method was proposed for ultrasensitive thrombin (TB) detection based on direct electrochemistry of highly loaded hemoglobin spheres-encapsulated platinum nanoparticles (PtNPs@Hb) as labels and electrocatalysts. The prepared PtNPs@Hb not only exhibited good biocompatibility, excellent electrocatalytic activity, but also presented redox activity of Hb. Thus, it was employed for the fabrication of aptasensor without any extraneous redox mediators, leading to a simple preparation process for the aptasensor. The high loading of Hb spheres as redox mediators could enhance the electrochemical signal. Importantly, the synergetic electrocatalytic behavior of Hb and PtNPs toward H2O2 reduction greatly amplified the electrochemical signal, resulting in the high sensitivity of aptasensor. Consequently, under optimal conditions, the designed aptasensor exhibited a lower detection limit of 0.05 pM and wide dynamic linear range from 0.15 pM to 40 nM for TB detection. Additionally, the proposed mediator-free and signal-amplified electrochemical aptasensor showed great potential in portable and cost-effective TB sensing devices.

Hemin on graphene nanosheets functionalized with flower-like MnO2 and hollow AuPd for the electrochemical sensing lead ion based on the specific DNAzyme.

Herein, integrated with DNAzyme highly specific to metal ions, hemin@reduced graphene oxide (hemin@rGO) functionalized with flower-like MnO2 and hollow AuPd (hAuPd-fMnO2-hemin@rGO) was used as electroactive probe and electrocatalyst to construct a universal platform for metal ion detection (lead ion Pb(2+) as the model). The proposed strategy with generality was mainly based on two aspects. Firstly, the designed probe not only showed high stability and excellent peroxidase-like activity originating from hemin, fMnO2 and hAuPd, but also possessed intrinsic redox performance from hemin, which resulted in the promotion of electron transfer and the enhancement of the response signal readout. Secondly, due to the introduction of Pb(2+), Pb(2+)-dependent DNAzyme bound in the electrode surface could be specifically identified and cleaved by Pb(2+), and the remained fragment (its supplementary sequence is a single-strand DNA S3) captured the nanocomposites S3-hAuPd-fMnO2-hemin@rGO by the hybridization reaction. Therefore, combined the cooperative catalysis of fMnO2, hAuPd and hemin to H2O2 reduction with highly specific interaction of Pb(2+)-dependent DNAzyme, the proposed Pb(2+) biosensor showed significant improvement of electrochemical analytical performance, which was involved in wide dynamic response in the range of 0.1pM-200nM, low detection limit of 0.034pM, high sensitivity and high specificity. This could facilitate the universal strategy to be a promising method for detection of other metal ions, only changing the corresponding DNAzyme specific to them.

Gold nanoparticle–graphene nanohybrid bridged 3-amino-5-mercapto-1,2,4-triazole-functionalized multiwall carbon nanotubes for the simultaneous determination of hydroquinone, catechol, resorcinol and nitrite

In this report, gold nanoparticle–graphene nanohybrids (Au–GR) and 3-amino-5-mercapto-1,2,4-triazole-functionalized multiwall carbon nanotubes (MWCNT–SH) were synthesized. And a novel hybrid material MWCNT–SH@Au–GR was obtained by the interaction between gold nanoparticles of two dimensional (2D) Au–GR and SH groups of 1D MWCNT–SH. Due to the synergistic effects between MWCNT–SH and Au–GR and excellent film forming ability of MWCNT–SH@Au–GR, the obtained MWCNT–SH@Au–GR was used as a modifier to fabricate a chemically modified electrode for the simultaneous determination of hydroquinone (HQ), catechol (CC), resorcinol (RC) and nitrite (NO2−). Scanning electron microscopy (SEM) was employed to characterize the morphology of MWCNT–SH@Au–GR. The electrochemical behavior of the sensor was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. Results showed that it was feasible to simultaneously measure HQ, CC, RC and NO2−. The linear response ranges for HQ, CC, RC and NO2− were 54.5–1250.5 μM, 11.0–126.0 μM, 43.5–778.5 μM and 86.0–7500.0 μM and the detection limits (S/N = 3) were 4.17 μM, 1.00 μM, 7.80 μM and 23.5 μM, respectively.

Dendritic structure DNA for specific metal ion biosensor based on catalytic hairpin assembly and a sensitive synergistic amplification strategy.

In this work, a sensitive electrochemical biosensing to Pb2+ was proposed based on the high specificity of DNAzymes to Pb2+. The response signal was efficiently amplified by the catalytic hairpin assembly induced by strand replacement reaction and the formation of dendritic structure DNA (DSDNA) by layer-by-layer assembly. Firstly, in the presence of Pb2+, the substrate strand (S1) of the Pb2+-specific DNAzymes was specifically cleaved by Pb2+. Secondly, one of the two fragments (rS1) introduced into the electrode surface was hybridized with a hairpin DNA (H1) and further replaced by another hairpin DNA (H2) by the hybridization reaction of H1 with H2. The released rS1 then induced the next hybridization with H1. After repeated cycles, the catalytic recycling assembly of H2 with H1 was completed. Thirdly, two bioconjugates of Pt@Pd nanocages (Pt@PdNCs) labeled with DNA S3/S4 and electroactive toluidine blue (Tb) (Tb-S3-Pt@PdNCs and Tb-S4-Pt@PdNCs) were captured onto the resultant electrode surface through the hybridization of S3 and H2, S3 and S4, resulting in the formation of DSDNA triggered by layer-by-layer assembly. This formed DSDNA greatly facilitated the immobilization of manganese(III) meso-tetrakis (4-N-methylpyridiniumyl)-porphyrin (MnTMPyP) as mimicking enzyme. Under the synergistic catalysis of Pt@PdNCs and MnTMPyP to H2O2 reduction, the effective signal amplification of the developed Pb2+ biosensor was achieved. As a result, the sensitive detection of the proposed electrochemical strategy for Pb2+ was greatly improved in the range of 0.1pM-200nM with a detection limit of 0.033pM.

A ‘signal on-off’ electrochemical peptide biosensor for matrix metalloproteinase 2 based on target induced cleavage of a peptide

In this work, a ‘signal on-off’ electrochemical peptide biosensor was developed for the determination of matrix metalloproteinase 2 (MMP-2) on the basis of target induced cleavage of a specific peptide. The prepared single-stranded DNA–porous platinum nanoparticles–peptide (S1–pPtNPs–P1) bioconjugates were employed as nanoprobes, where the specific peptide (P1, biotin–Gly–Pro–Leu–Gly–Val–Arg–Gly–Lys–Gly–Gly–Cys) was used as a cleavage-sensing element, offering the capability of ‘on-off’ electrochemical signalling for the target MMP-2. As for the construction of the biosensor, S1–pPtNPs–P1 was immobilized on the electrode surface through the conjunction of biotin–streptavidin. Then, hybridization chain reaction (HCR) was triggered to embed the electroactive thionine (Thi). The pPtNPs could effectively catalyze the decomposition of added H2O2, resulting in the electrochemical signal of Thi being enhanced significantly (‘signal on’ state). Upon sensing cleavage with MMP-2, pPtNPs and eletroactive Thi left the electrode surface, leading to an observable decrease in the electrochemical signal of Thi (‘signal off’ state). Compared with other methods of detecting MMP-2, the proposed ‘signal on-off’ electrochemical peptide biosensor exhibited an improved sensitivity with a detection limit of 0.32 pg mL−1 and wide linear range from 1 pg mL−1 to 10 ng mL−1.

Conjugates of graphene oxide covalently linked ligands and gold nanoparticles to construct silver ion graphene paste electrode

We reported on the synthesis and application of ionophore-gold nanoparticle conjugates in Ag(+) graphene paste electrode. Ionophore was a novel graphene oxide nanosheets (NGO) covalently grafted 2-thiophenecarboxylic (TPC) hybrid material. The hybrid material NGO-TPC decorated with gold nanoparticles was used as both a receptor and an ion-to-electron transducer to fabricate Ag(+) graphene paste electrode. The developed electrode was highly selective to Ag(+) over other tested cations and exhibited an excellent Nernstian slope of 59.3 mV dec(-1) ranging from 8.4×10(-7) to 1.0×10(-) M with a detection limit of 6.3×10(-7) M. Moreover, it also showed a fast response time and a long lifetime. Importantly, the new method of immobilizing ligands on NGO nanosheets to construct electrode successfully solved the universal problem of the electrode components loss from ion-selective electrode.

Glucose oxidase-initiated cascade catalysis for sensitive impedimetric aptasensor based on metal-organic frameworks functionalized with Pt nanoparticles and hemin/G-quadruplex as mimicking peroxidases

Based on cascade catalysis amplification driven by glucose oxidase (GOx), a sensitive electrochemical impedimetric aptasensor for protein (carcinoembryonic antigen, CEA as tested model) was proposed by using Cu-based metal-organic frameworks functionalized with Pt nanoparticles, aptamer, hemin and GOx (Pt@CuMOFs-hGq-GOx). CEA aptamer loaded onto Pt@CuMOFs was bound with hemin to form hemin@G-quadruplex (hGq) with mimicking peroxidase activity. Through sandwich-type reaction of target CEA and CEA aptamers (Apt1 and Apt2), the obtained Pt@CuMOFs-hGq-GOx as signal transduction probes (STPs) was captured to the modified electrode interface. When 3,3-diaminobenzidine (DAB) and glucose were introduced, the cascade reaction was initiated by GOx to catalyze the oxidation of glucose, in situ generating H2O2. Simultaneously, the decomposition of the generated H2O2 was greatly promoted by Pt@CuMOFs and hGq as synergistic peroxide catalysts, accompanying with the significant oxidation process of DAB and the formation of nonconductive insoluble precipitates (IPs). As a result, the electron transfer in the resultant sensing interface was effectively hindered and the electrochemical impedimetric signal (EIS) was efficiently amplified. Thus, the high sensitivity of the proposed CEA aptasensor was successfully improved with 0.023pgmL-1, which may be promising and potential in assaying certain clinical disease related to CEA.

A sensitive impedimetric platform biosensing protein: Insoluble precipitates based on the biocatalysis of manganese(III) meso-tetrakis (4-N-methylpyridiniumyl)-porphyrinin in HCR-assisted dsDNA

In this study, a sensitive biosensing interface for protein was reported based on nonconductive insoluble precipitates (IPs) by the biocatalysis of manganese(III) meso-tetrakis (4-N-methylpyridiniumyl)-porphyrin (MnTMPyP), which was intercalated into formed double-strand DNA (dsDNA) scaffold triggered by hybridization chain reaction (HCR). In the proposed impedimetric aptasensor, carcinoembryonic antigen (CEA) and its aptamer were used as testing model. PtPd nanowires (PtPdNWs) with large surface area and superior conductivity were employed as nanocarriers to greatly immobilize biomolecules (e.g. CEA aptamers). Then, two DNA hairpins H1 and H2 were introduced to trigger HCR with the assistance of DNA initiator, resulting in the formation of a long dsDNA scaffold. Meanwhile, mimicking enzyme MnTMPyP molecules were embedded into the resultant dsDNA, in situ generating the complex MnTMPyP-dsDNA with peroxidase-like activity. Under the biocatalysis of MnTMPyP-dsDNA, 3,3-diaminobenzidine (DAB) was oxidized to form nonconductive IPs. As a result, the electron transfer between electrode interface and redox probe was vastly hindered, leading to the significant amplification of electrochemical impedimetric signal. So, greatly improved analytical performances of the proposed aptasensor were achieved with a detection limit as low as 0.030pgmL(-1). And the successful assay of CEA in human serum samples enabled the developed biosensing platform to have promising potential in bioanalysis.

A label-free and sensitive electrochemical aptasensor for thrombin based on the direct electron transfer of hemin and hemin@rGO nanosheets as the signal probe

In this work, a label-free electrochemical aptasensor for sensitive detection of thrombin was fabricated and characterized. This aptasensor was based on the direct electron transfer of hemin and hemin functionalized reduced graphene oxide hybrid nanosheets (hemin@rGO) as the signal probe. Hemin@rGO with intrinsic peroxidase-like activity could catalyze the reaction of the peroxidase substrate because of the catalytic ability of hemin attached on the graphene surface through π–π interactions. The greatly enhanced sensitivity of the as-prepared aptasensor for thrombin was based on an effective signal amplification strategy. Firstly, the synthesized hemin@rGO nanosheets not only provided a large conductive interface, but also exhibited excellent redox activity with avoidance of an extra electroactive mediator. Secondly, the electrodeposition of Pt nanoparticles (PtNPs) on the resultant electrode surface effectively promoted the electron transfer and amplified the electrochemical response. Thirdly, further enhanced sensitivity was achieved by the outstanding catalytic performance of the horseradish peroxidase as the blocking reagent. On the basis of such a signal amplification strategy, the direct and facile electrochemical aptasensor showed superior electrocatalytic efficiency toward H2O2, and sensitively responded to 0.45 pM thrombin with a linear calibration range from 1 pM to 50 nM. So, the proposed detecting platform for thrombin could be promising for clinical analysis and assays.