As an acidic glycoprotein, carcinoembryonic antigen (CEA) is of great value as a broad-spectrum tumor marker in the differential diagnosis and surveillance of malignant tumors. In this work, we report an electrochemical aptasensor for the ultrasensitive and highly selective detection of CEA, taking advantage of the dual amplification by the boronate affinity-assisted electrochemically controlled atom transfer radical polymerization (BA-eATRP). Specifically, the BA-eATRP-based electrochemical aptasensing of CEA involves the capture of target antigens by nucleic acid aptamers, the covalent crosslinking of ATRP initiators to CEA antigens via the selective interactions between the phenylboronic acid (PBA) group and the cis-diol group of the monosaccharide residues, and the collection of the ferrocene (Fc) reporters via the eATRP of ferrocenylmethyl methacrylate (FcMMA). As CEA is decorated with hundreds of cis-diol groups, the BA-based crosslinking can result in the labeling of each CEA with hundreds of ATRP initiators; furthermore, the eATRP of FcMMA results in the surface-initiated growth of long-chain ferrocenyl polymers, leading to the tethering of each ATRP initiator-conjugated site with hundreds to thousands of Fc reporters. Such that, the BA-eATRP can result in the efficient labeling of each CEA with a plenty of Fc reporters. Under the optimized conditions, the BA-eATRP-based strategy enables the highly selective aptasensing of CEA at a concentration as low as 0.34 pg·mL⁻¹, with a linear range of 1.0−1,000 pg·mL⁻¹. Besides, this aptasensor has been successfully applied to the quantitative analysis of CEA in human serum. The BA-eATRP-based electrochemical aptasensor is cost-effective and simple in operation, holding broad application prospect in the ultrasensitive and highly selective detection of CEA.

Pathogenic bacteria have been throwing great threat on human health for thousands of years. Their real-time monitoring is in urgent need as it could effectively halt the spread of pathogenic bacteria and thus reducing the risk to human health. Up till now, diverse technologies such as electrochemistry, optics, piezoelectricity and calorimetry have been developed for bacteria sensing. Therein, electrochemical impedance spectroscopy (EIS)-based sensors show great potential in point-of-care bacterial analysis because of their low-cost, short read-out time, good reproducibility, and portable equipment construction. In this review, we will primarily summarize the typical applications of electrochemical impedance technology in bacteria sensing based on different electrodes in the last three years. As we know, the electrode materials play an extremely important role in the construction of EIS-based sensors because not only the immobilization of bio-recognition elements for bacteria, but also the sensitivity, economical efficiency and portability of the as-prepared sensors are mainly determined by the electrode materials. Therefore, in order to provide new researchers a clear preparation process for EIS-based sensors fabricated with different electrodes, we try to classify the EIS-based sensors according to the different electrode platforms. Moreover, present difficulties, future directions and perspectives for their applications are also discussed. It can provide guidance in future study of novel EIS-based sensors for rapid, sensitive and accurate sensing of diverse pathogenic bacteria.

Development of ultrasensitive, highly accurate and selective immunosensors is significant for the early diagnosis, screening, and monitoring of diseases. Electrochemical and electrochemiluminescent (ECL) immunoassays have both attracted great attention and become a current research hotspot due to their advantages such as good stability, high sensitivity and selectivity, wide linear range, and good controllability. Metal-organic frameworks (MOFs), as a new class of porous crystalline materials, have been widely applied in electrochemical and ECL immunosensors owing to their large specific surface area, good chemical stability, as well as adjustable pore size and nanoscale framework structures. Various MOF nanomaterials with different properties for the development of high-performance electrochemical and ECL immunosensors can be achieved, because they can be applied as sensitive platforms for immobilizing biological recognition molecules, enriching the trace analytes and signal molecules, amplifying the signal and enhancing the sensitivity of the electrochemical or ECL immunoassays. This review summarizes various types of MOFs-based immunosensors and their assays application, in which MOFs act as electrode matrices, signal probes (either as electroactive labels or as emitter labels), carriers or catalytic labels for sensitive electrochemical and ECL detections. Moreover, challenges and future opportunities for the development of the functionalized MOFs are discussed to provide a guidance on the design and fabrication of high-performance MOFs-based immunosensors in the future.

Potassium ion (K⁺) is widely involved in several physiopathological processes, and its abnormal changes are closely related to the occurrence of brain diseases of cerebral ischemia. In vivo acquirement of K⁺ variation is significant to understand the roles of K⁺ playing in brain functions. A microelectrode based on single-stranded DNA aptamers was developed for highly selective detection of K⁺ in brain, in which the aptamer probes were designed to contain an aptamer part for specific recognition of K⁺, an alkynyl group used for stable confinement of aptamer probe on the gold surface, and an electrochemical redox active ferrocene group to generate current response signal. The response range of the microelectrodes could be rationally tuned by varying the chain length of the aptamer probe. The optimized electrode, LAC, displayed high selectivity for in vivo detection of K⁺, and suitable linear range from 10 μmol·L^-1-10 mmol·L^-1, which could fulfill the requirement of K⁺ detection in brain. Eventually, the microelectrodes were successfully applied for the detection of K⁺ in the living mouse brains followed by hypoxic.

Crafting charge transfer channels at titanium dioxide (TiO₂) based photoanodes remain a pressing bottleneck in solar-to-chemical conversion technology. Despite the tremendous attempts, TiO₂ as the promising photoanode material still suffers from sluggish charge transport kinetics. Herein, we propose an assembly strategy that involves the axial coordination grafting metalloporphyrin-based photosensitizer molecules (MP) onto the surface-modified TiO₂ nanorods (NRs) photoanode, forming the composite MP/TiO₂ NRs photoelectrode. As expected, the resulted unique MP_B/TiO₂ NRs photoelectrode displays significantly improved photocurrent density as compared to TiO₂ NRs alone and MP_A/TiO₂ NRs photoelectrode. Scanning photoelectrochemical microscopy (SPECM) and intensity modulated photocurrent spectroscopy (IMPS) were employed to systematically evaluate the continuous photoinduced electron transfer (PET) dynamics for MP/TiO₂ NRs photoelectrode. According to the data fitting, it is found that the photoelectron transfer rate (k_eff) constant for the MP_B/TiO₂ NRs is about 2.6 times higher than that for the pure TiO₂ NRs under light irradiation. The high kinetic constant for the MP_B/TiO₂ NRs was ascribed to that the conjugated molecules MP_B of D-A structure can effectively accelerate intramolecular electrons transfer as well as promote electrons taking part in the reduction reaction of I₃^- to I^- in the novel charge transfer channel. The results demonstrated in this study are expected to shed some light on investigating the mechanism in the charge transfer process of artificial photosynthesis and constructing efficient photoelectrodes.

Plasmonic nanoparticles such as Au and Ag with localized surface plasmon resonance (LSPR) property exhibit unique scattering and absorption features. The plasmonic scattering and absorption bands are mainly located at visible light region which can be easily applied in visual detections. By modulating the size, shape and composition of gold and silver colloid solutions, plenty of colorimetric methods have been designed for the detection of metal ions, biomolecules and environmental contaminants. For many years, the LSPR-based measurements are implemented in reagent tubes. Since 2000, the plasmon resonance scattering (PRS) light of metal nanoparticles captured by dark-field microscopy enables the investigation at the nanoscale dimension. Mono-dispersed nanoparticles under a dark-field microscope showed distinct scattering light spots, like colorful stars in the dark sky. The PRS light of a single nanoparticle opens a new way for ultra-sensitive sensing which eliminates the average effects in bulk and provides more accurate reaction information. Thus, individual nanoparticles with specific scattering colors are excellent nanoprobes in the applications of biology, physics, and chemistry. In this review, the plasmonics based colorimetric nanosensors are presented. Particularly, the application of in-situ PRS in the dynamically monitoring of electrocatalytic reactions is highlighted. We firstly introduce a short history of the discovery and development of plasmonic nanoparticles from the ancient artwork to the modern characterization techniques. Some factors including morphology, and dielectric constants that are correlated to the LSPR bands and scattering light colors are listed. Secondly, we demonstrate the use of single plasmonic nanoparticles as visualized color-coded nanoprobes. As the morphology of particles has strong effect on the PRS light, elegant sensors have been conceived by the etching and growth of nanoparticles with different sizes and shapes. On the other hand, the real-time monitoring of particle structure evolution could also reveal the mechanism of the material fabrication at the nanoscale. In addition, core-satellite nanostructures with various linkers are proposed as ultra-sensitive sensors according to the inter-particle coupling effect. Subsequently, we summarize several advanced techniques for nanoscale signal extraction and amplifications. For instance, to expand the application of colorimetric nanosensors, converting the colors into RGB values could clearly distinguish the subtle color changes. Combining with high-throughput signal processing method, thousands of nanoparticles can be rapidly analyzed, which can greatly enhance the measurement efficiency. Except the PRS color, the PRS intensity could also provide abundant information and is easier to be captured. A facile method by converting the PRS intensity of single nanoparticles into visible colors is presented, which is mighty suitable for the in-situ monitoring of fast electrochemical process with high time resolution.