Fractal gold modified electrode for ultrasensitive thrombin detection

Amorphous mixed-valent Mn-containing nanozyme with cocklebur-like morphology for specific colorimetric detection of cancer cells via Velcro effects

Designing nanozymes with excellent catalytic activity through valence state engineering and defect engineering is a widely applicable strategy. However, their development is hindered by the complexity of the design strategies. In this work, we employed a simple calcination method to regulate the valence of manganese and crystalline states in manganese oxide nanozymes. The oxidase-like activity of the nanozymes was found to benefit from a mixed valence state dominated by Mn (III). And the amorphous structure with more active defect sites significantly enhanced the catalytic efficiency. Moreover, we demonstrated that amorphous mixed-valent Mn-containing (amvMn) nanozymes with unique cocklebur-like biomimetic morphology achieved specific binding to cancer cells through the Velcro effects. Subsequently, the nanozymes mediated TMB coloration through their oxidase-like activity, enabling the colorimetric detection of cancer cells. This work not only provides guidance for optimizing nanozyme performance, but also inspire the development of equipment-free visual detection methods for cancer cells.

Bioinspired superwettable electrodes towards electrochemical biosensing

Superwettable materials have attracted much attention due to their fascinating properties and great promise in several fields. Recently, superwettable materials have injected new vitality into electrochemical biosensors. Superwettable electrodes exhibit unique advantages, including large electrochemical active areas, electrochemical dynamics acceleration, and optimized management of mass transfer. In this review, the electrochemical reaction process at electrode/electrolyte interfaces and some fundamental understanding of superwettable materials are discussed. Then progress in different electrodes has been summarized, including superhydrophilic, superhydrophobic, superaerophilic, superaerophobic, and superwettable micropatterned electrodes, electrodes with switchable wettabilities, and electrodes with Janus wettabilities. Moreover, we also discussed the development of superwettable materials for wearable electrochemical sensors. Finally, our perspective for future research is presented.

Unusual Nanofractal Microparticles for Rapid Protein Capture and Release

Ion exchange porous microparticles are widely used for protein separation, but their totally porous structure often leads to slow diffusion rate and long separation time. Here unusual nanofractal microparticles synthesized by a strategy of electrostatic interaction regulated emulsion interfacial polymerization are demonstrated that exhibit excellent capability of rapid protein capture, release, and separation. The growth of nanostructures at nanofractal microparticle surface can be controlled by changing electrostatic repulsion between ion groups from weak to strong. The nanofractal microparticles provide a 3D contact model between ion groups and proteins, enable fast protein diffusion rate at initial capture and release stage, and realize rapid and efficient separation of similarly sized proteins as a proof of concept, superior to porous microparticles. This strategy offers an effective and general way for the synthesis of microparticles towards rapid and efficient separation in various fields of biomedicine, environment, and food.

Target-induced photocurrent-polarity switching: a highly selective and sensitive photoelectrochemical sensing platform

Based on target-induced photocurrent-polarity switching, a highly selective and sensitive photoelectrochemical sensing platform was developed.

Self-standing aptamers by an artificial defect-rich matrix

The classical alkanethiol post-passivation can prevent nonspecific binding of nucleotide bases onto supporting substrates and help aptamers transition from a "lying down" to a "standing up" orientation. However, the surface probes display lower binding affinity towards targets than those in bulk solutions due to unsatisfied hybridization spaces on the alkanethiol passivated substrate. To overcome this challenge, an artificial defect-rich matrix possessing an aptamer "self-standing" property created by chemical lift-off lithography (CLL) is demonstrated. This approach provided artificial defects on a hydroxyl-terminated alkanethiol self-assembled monolayer (SAM), which allowed the insertion of thiolated aptamers. The diluted surface molecular environment assisted aptamers not only to "self-stand" on the surface, but also to separate from each other, providing a suitable surface aptamer density and sufficient space for capturing targets. With this approach, the binding affinity of the aptamer towards a target was comparable to solution-type probes, showing higher recognition efficiency than that in conventional methods.

Stimulus-response click chemistry based aptamer-functionalized mesoporous silica nanoparticles for fluorescence detection of thrombin

In most aptamer based stimulus response mesoporous silica nanoparticles (MSN) systems, the aptamer is modified on the MSN via electrostatic interaction, however leakage might exist after a certain time in the system and hence the stability is not good. In this study, the pores of MSN were capped by aptamer through click chemistry reaction for the first time and the system was then employed to develop a fluorescence biosensor. Specifically, the aptamer of the target (thrombin in this study) was hybridized with its complementary DNA (which was initially modified with alkyne at the terminal) to form a double strand DNA (dsDNA) firstly, and then this dsDNA was modified on N₃ modified MSN via Cu(I) catalyzed alkyne-azide cycloaddition reaction. The guest molecules (fluorescein) were blocked in the pores of the MSN with high efficiency and nearly no leakage was detected. Upon the introduction of thrombin, thrombin specifically recognized its aptamer, so aptamer released from the MSN; and the single strand DNA(ssDNA) left could not cap the pores of the MSN efficiently and hence caused the releasing of fluorescein into the solution. The enhanced fluorescence intensity of the system has a good linear relationship with the thrombin concentration in the range of 50-1000ngmL^-1 with a detection limit of 28.46ngmL^-1. The proposed biosensor has been successfully applied to detect thrombin in serum samples with high selectivity. The same strategy can be applied to develop biosensors for different targets by changing the adopted aptamer.

Enhancement of dielectrophoresis using fractal gold nanostructured electrodes

Dielectrophoretic motions of Saccharomyces cerevisiae (yeast) cells and colloidal gold are investigated using electrochemically modified electrodes exhibiting fractal topology. Electrodeposition of gold on electrodes generated repeated patterns with a fern-leaf type self-similarity. A particle tracking algorithm is used to extract dielectrophoretic particle velocities using fractal and planar electrodes in two different medium conductivities. The results show increased dielectrophoretic force when using fractal electrodes. Strong negative dielectrophoresis of yeast cells in high-conductivity media (1.5 S/m) is observed using fractal electrodes, while no significant motion is present using planar electrodes. Electrical impedance at the electrode/electrolyte interface is measured using impedance spectroscopy technique. Stronger electrode polarization (EP) effects are reported for planar electrodes. Decreased EP in fractal electrodes is considered as a reason for enhanced dielectrophoretic response.

Rough Gold Electrodes for Decreasing Impedance at the Electrolyte/Electrode Interface

Electrode polarization at the electrolyte/electrode interface is often undesirable for bio-sensing applications, where charge accumulated over an electrode at constant potential causes large potential drop at the interface and low measurement sensitivity. In this study, novel rough electrodes were developed for decreasing electrical impedance at the interface. The electrodes were fabricated using electrochemical deposition of gold and sintering of gold nanoparticles. The performances of the gold electrodes were compared with platinum black electrodes. A constant phase element model was used to describe the interfacial impedance. Hundred folds of decrease in interfacial impedance were observed for fractal gold electrodes and platinum black. Biotoxicity, contact angle, and surface morphology of the electrodes were investigated. Relatively low toxicity and hydrophilic nature of the fractal and granulated gold electrodes make them suitable for bioimpedance and cell electromanipulation studies compared to platinum black electrodes which are both hydrophobic and toxic.

Three label-free thrombin aptasensors based on aptamers and [Ru(bpy)2(o-mopip)]2+

The label free aptasensor based on an aptamer pair and [Ru(bpy)₂(o-mopip)]²⁺–GO has been successfully applied to sandwich assays for thrombin detection.

Self-interconnecting Pt nanowire network electrode for electrochemical amperometric biosensor

One-dimensional Pt nanostructures are of considerable interest for the development of highly stable and sensitive electrochemical sensors. This paper describes a self-interconnecting Pt nanowire network electrode (PtNNE) for the detection of hydrogen peroxide (H2O2) and glucose with ultrahigh sensitivity and stability. The as-prepared PtNNE consists of polycrystalline nanowires with high-index facets along the side surface which provides more active surface atoms on kinks and steps, those ultralong nanowires being interconnected with each other to form a free-standing network membrane. The excellent structural features of the PtNNE promoted its performance as a Pt-based electrochemical sensor both in terms of electrocatalytic activity and stability. Amperometric measurements towards hydrogen peroxide were performed; the PtNNE sensor showed an extremely high sensitivity of 1360 μA mM(-1) cm(-2). This excellent sensitivity is mainly attributed to the high-index facets of the nanowires resulting in their superior electrocatalytic activity towards H2O2, and the interconnected nanowire network forming an "electron freeway" transport model, which could provide multiple electron pathways and fast electron transport on the electrode, leading to rapid reaction and sensitive signal detection. The as-prepared PtNNE also holds promise as an oxidase-based biosensor. As a proof of concept, a PtNNE-based glucose biosensor also showed an outstanding sensitivity as high as 114 μA mM(-1) cm(-2), a low detection limit of 1.5 μM, and an impressive detection range from 5 μM to 30 mM.

An aptamer-based single particle method for sensitive detection of thrombin using fluorescent quantum dots as labeling probes

In this study, an aptamer-based single particle method was developed for the thrombin detection in human serum samples using fluorescence correlation spectroscopy (FCS). In this method, quantum dots (QDs) were used as the fluorescent probes and thrombin-binding aptamer (TBA) was used as molecular recognition unit. When two QDs probes labeled with TBA (QD-TBA1 and QD-TBA2) are mixed in a sample containing thrombin targets, the binding of targets will cause QDs to form dimers (or oligomers) with bigger sizes, which leads to the nearly double increase in the characteristic diffusion time of QDs in the detection volume of FCS. FCS method can detect the change in the characteristic diffusion time of QDs. Firstly, the diffusion and blinking behaviors of QD-TBA probes in the presence of thrombin were investigated by FCS and total internal reflection fluorescence microscopy (TIRFM) imaging system, and the experimental results documented that QD-TBAs were bound together with "one-by-one" structure when thrombin were added into the solution. And then, the assay conditions were optimized in order to improve the sensitivity and specificity of this method. Under the optimized conditions, the linear range of the method is from 5.0 nM to 500 nM of thrombin, and the limit of detection is about 2.6 nM. Finally, this method was applied to homogeneous determination of thrombin in human serum samples.

Facile fabrication of a three-dimensional gold nanowire array for high-performance electrochemical sensing

A three-dimensional gold nanowire array (3D GNA) was successfully prepared with a facile template-assisted approach, in order to construct an ultrasensitive electrochemical biosensor.

Thrombin detection using a piezoelectric aptamer-linked immunosorbent assay

The development of diagnostic assays using highly targeted specific aptamers with existing detection platforms has been an endeavor with few opportunities until now. Many current commercially available diagnostic platforms make use of detection systems employing capture agents composed of modified antigen-specific antibodies coupled with a variety of detection modalities, including radioimmunoassays, fluorescence-based detection assays, electro/chemiluminescence assays, and immunoradiometric assays. In the studies presented here, a novel frequency-modulating technology from BioScale called Acoustic Membrane MicroParticle (AMMP) detection was used to demonstrate a sensitive and reproducible method of incorporating aptamers as capture and detection agents. The method provides a robust and rapid detection of thrombin in human serum while also eliminating the labor-intensive efforts of Western blot analysis and is not affected by the interfering substances found in serum that often affect optical-based detection systems. In addition, we have demonstrated, for the first time, the adaptation of the AMMP platform to exploit aptamers against a clinically relevant target. The AMMP platform is an ideal medium for using aptamers in commercial assay development for application in a clinical setting.

Programmable fractal nanostructured interfaces for specific recognition and electrochemical release of cancer cells

Topographic recognition of cancer cells is triggered by fractal gold nanostructures (FAuNSs), leading to dramatically enhanced recognition capability and efficient release of cancer cells with little damage. The unique characteristic of FAuNSs is the similar fractal dimension of their surface and that of a cancer cell. The design of fractal nanostructures will open up opportunities for functional design of bio-interfaces for highly efficient recognition and release of disease-related rare cells, which will improve detection in a clinical environment.

Space-confined fabrication of silver nanodendrites and their enhanced SERS activity

Here we report a controllable method based on electrodeposition to fabricate Ag nanodendrites (NDs) on a microwell patterned electrode. The microwell patterns on the ITO electrode are fabricated via the microcontact printing technique. By varying the microwell size and electrodeposition time, the morphology of metal deposits on the microwell patterned ITO electrode can be tuned from boulders to dendrites. At the edge of the microwells, the current density was strengthened, which incurs rapid nucleation. The nucleus develops into dendrites because of Mullins-Sekerka instability. However, only boulders were observed at the center of microwells. By reducing the size of the microwells, only NDs were fabricated due to the edge effect. On the basis of understanding the underlying mechanism for dendritic growth in a confined space, our method is used for fabricating other noble metal (Au, Pt) nanodendrites. The controllable synthesis of Au and Pt NDs indicates the universality of this method. Compared with Ag film obtained from electron beam evaporation, the as-prepared Ag NDs exhibit highly enhanced surface-enhanced Raman scattering (SERS) sensitivity when they are used to detect rhodamine 6G (R6G). This approach provides a very controllable, reliable and general way for space-confined fabricating the noble metal nanodendrite arrays which show great promise in catalysis, sensing, biomedicine, electronic and magnetic devices.

Aptamer-based sensing for thrombin in red region via fluorescence resonant energy transfer between NaYF₄:Yb,Er upconversion nanoparticles and gold nanorods

In this work, we design a FRET system for sensitive and selective determination of thrombin in red region, in which NaYF₄:Yb,Er upconversion nanoparticles (UCNPs) act as donor and gold nanorods (Au NRs) act as acceptor. NaYF₄:Yb,Er UCNPs with a strong emission at 661 nm were successfully synthesized by tuning the doped ions ratio. Carboxyl-functionalized NaYF₄:Yb,Er UCNPs and Au NRs were then prepared and conjugated with the thrombin aptamers, respectively. The fluorescence emission band of NaYF₄:Yb,Er UCNPs (λ(max)=661 nm) highly overlaps with the absorption band of Au NRs(λ(max)=666 nm), which benefits from the large tunability of the spectrum band of Au NRs. A FRET system was then formed when thrombin was added to the mixture of NaYF₄:Yb,Er UCNPs and Au NRs, which were both modified thrombin aptamers. The fluorescence quenching efficiency of NaYF₄:Yb,Er UCNPs was increased in a thrombin concentration-dependent manner, which built the principle of thrombin quantification. The linear range was 2.5-90 nM in an aqueous buffer, and 3.75-112.5 nM in spiked human serum samples for thrombin. It also demonstrates a high selectivity to other biological species due to the specific binding. The measurement of thrombin in human plasma is satisfying, suggesting that the FRET system is of practical value in a complex biological sample matrix in red region.

Trace and label-free microRNA detection using oligonucleotide encapsulated silver nanoclusters as probes

A simple, sensitive, and label-free method for microRNA (miRNA) biosensing was described using oligonucleotide encapsulated silver nanoclusters (Ag-NCs) as effective electrochemical probes. The functional oligonucleotide probe integrates both recognition sequence for hybridization and template sequence for in situ synthesis of Ag-NCs, which appears to possess exceptional metal mimic enzyme properties for catalyzing H(2)O(2) reduction. The miRNA assay employs gold electrodes to immobilize the molecular beacon (MB) probe. After the MB probe subsequently hybridizes with the target and functional probe, the oligonucleotide encapsulated Ag-NCs are brought to the electrode surface and produce a detection signal, in response to H(2)O(2) reduction. An electrochemical miRNA biosensor down to 67 fM with a linear range of 5 orders of magnitude was obtained. Meanwhile, the MB probe allows the biosensor to detect the target with high selectivity. The Ag-NCs-based approach provides a novel avenue to detect miRNA with high sensitivity and selectivity while avoiding laborious label and signal amplification. It is convinced that rational introduction of signal amplification strategy to the Ag-NCs-based bioanalysis can further improve the sensitivity. To our best knowledge, this is the first application of the electrocatalytic activity of Ag-NCs in bioanalysis, which would be attractive for genetic analysis and clinic biomedical application.