Sensitive and selective monitoring of sialic acid (SA) in cerebral nervous system is of great importance for studying the role that SA plays in the pathological process of Alzheimer’s disease (AD). In this work, we first reported an electrochemical biosensor based on a novel stimuli-responsive copolymer for selective and sensitive detection of SA in mouse brain. Notably, through synergetic hydrogen-bonding interactions, the copolymer could translate the recognition of SA into their conformational transition and wettability switch, which facilitated the access and enrichment of redox labels and targets to the electrode surface, thus significantly improving the detection sensitivity with the detection limit down to 0.4 pM. Besides amplified sensing signals, the proposed method exhibited good selectivity toward SA in comparison to potential interference molecules coexisting in the complex brain system due to the combination of high affinity between phenylboronic acid (PBA) and SA and the directional hydrogen-bonding interactions in the copolymer. The electrochemical biosensor with remarkable analytical performance was successfully applied to evaluate the dynamic change of SA level in live mouse brain with AD combined with in vivo midrodialysis. The accurate concentration of SA in different brain regions of live mouse with AD has been reported for the first time, which is beneficial for progressing our understanding of the role that SA plays in physiological and pathological events in the brain.Keywords: Alzheimer’s disease; electrochemical sensor; microdialysis; sialic acid; stimuli-responsive polymer;

Glucose and pH are two important indicators of diabetes mellitus. However, their dynamic changes at the same time in brain are still not clear, mainly due to a lack of a single biosensor capable of simultaneous quantification of two species in a live rat brain. In this work, a selective and sensitive ratiometric electrochemical biosensor was developed for simultaneously quantifying glucose and pH using both current and potential outputs in a rat brain of diabetic model. Here, glucose oxidase was first employed as a specific recognition element for both glucose and pH because the active center (FAD) could undergo a 2H+/2e– process. Moreover, an insensitive molecule toward pH and glucose was used as an inner-reference element to provide a built-in correction to improve the accuracy. The ratio between the oxidation peak current density of glucose and that of ABTS gradually increased with increasing concentration of glucose, and showed a good linearity in the range of 0.3–8.2 mM. Meanwhile, the midpotential difference between glucose oxidase and 2,2′-azino-bis (3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) positively shifted with pH decreasing, leading to accurate determination of pH in the linear range of 5.67–7.65. Thus, combined with the unique properties of carbon fiber microelectrode, including easy to insert and good biocompatibility, the developed single biosensor was successfully applied to detect pH and glucose at the same time in hippocampus, striatum, and cortex in a live rat brain of diabetic model.

Hurdles of nanopore modification and characterization restrain the development of glass capillary-based nanopore sensing platforms. In this article, a simple but effective biomimetic mineralization method was developed to decorate glass nanopore with a thin film of bovine serum albumin-protected Au nanocluster (BSA-Au NC). The BSA-Au NC film emitted a strong red fluorescence whereby nondestructive characterization of Au film decorated at the inner surface of glass nanopore can be facilely achieved by a fluorescence microscopy. Besides, the BSA molecules played dual roles in the fabrication of functionalized Au thin film in glass nanopore: they not only directed the synthesis of fluorescent Au thin film but also provided binding sites for recognition, thus achieving synthesis-modification integration. This occurred due to the ionized carboxyl groups (-COO–) of a BSA coating layer on Au NCs which can interacted with arginine (Arg) via guanidinium groups. The added Arg selectively led to the change in the charge and ionic current of BSA-Au NC film-decorated glass nanopore. Such ionic current responses can be used for quantifying Arg with a detection limit down to 1 fM, which was more sensitive than that of previous sensing systems. Together, the designed method exhibited great promise in providing a facile and controllable solution for glass nanopore modification, characterization, and sensing.

Intracellular pH plays a vital role in cell biology, including signal transduction, ion transport and homeostasis. Herein, a ratiometric fluorescent silica probe was developed to detect intracellular pH values. The pH sensitive dye fluorescein isothiocyanate isomer I (FITC), emitting green fluorescence, was hybridized with reference dye rhodamine B (RB), emitting red fluorescence, as a dual-emission fluorophore, in which RB was embedded in a silica core of ∼40 nm diameter. Moreover, to prevent fluorescence resonance energy transfer between FITC and RB, FITC was grafted onto the surface of core–shell silica colloidal particles with a shell thickness of 10–12 nm. The nanoprobe exhibited dual emission bands centered at 517 and 570 nm, under single wavelength excitation of 488 nm. RB encapsulated in silica was inert to pH change and only served as reference signals for providing built-in correction to avoid environmental effects. Moreover, FITC (λem = 517 nm) showed high selectivity toward H+ against metal ions and amino acids, leading to fluorescence variation upon pH change. Consequently, variations of the two fluorescence intensities (Fgreen/Fred) resulted in a ratiometric pH fluorescent sensor. The specific nanoprobe showed good linearity with pH variation in the range of 6.0–7.8. It can be noted that the fluorescent silica probe demonstrated good water dispersibility, high stability and low cytotoxicity. Accordingly, imaging and biosensing of pH variation was successfully achieved in HeLa cells.

•A “synthesis-modification integration” strategy was used to fabricate nanoprobe.•Aminothiols has been detected based on the inner filter effect principle.•The fluorescent assay featured theoretical simplicity and low technical demands.•The probe provided good recovery results (96–104%) in the spike serum samples.A simple, yet efficient fluorescent method for detecting biological aminothiols has been developed based on the inner filter effect principle that utilizes graphene quantum dots (GQDs) as the donor and aminothiol-Pb2+ complex as the absorber. Well-defined diethanol amine modified graphene quantum dots (GQD-DEA) were first synthesized by a “synthesis-modification integration” strategy. Then, the addition of aminothiols can bind with Pb2+ and displace it from the surface of preformed GQD-DEA-Pb2+, leading to the formation of aminothiol-Pb2+ complex. Due to the complementary overlap between the excitation band of GQD-DEA and the absorption band of aminothiol-Pb2+ complex, the fluorescence of GQDs was quenched, thereby a turn-off fluorescent assay for the determination of aminothiols via the inner filter effect was constructed. This strategy enabled cost-effective and selective detection of aminothiols with theoretical simplicity and low technical demands. Moreover, the fluorescent probe offered high selectivity for aminothiol due to the strong binding of aminothiol with Pb2+ in comparison with other amino acids and the inner filter effect provided by thiol-Pb2+ complex. Under the optimum conditions, the linear concentration ranges were 5 × 10−5–6 × 10−4 M for cysteine, 5 × 10−5–1 × 10−3 M for homocysteine, 1 × 10−4–2 × 10−3 M for glutathione, respectively.

Signal amplification of chiral interaction is a much needed task for sensing of enantiomers due to nearly identical chemical and physical properties of the chiral isomers. In this article, we established an electrochemical chiral sensing method with high sensitivity and selectivity for monosacharrides based on the stimuli-responsive copolymer/graphene hybrid-modified screen-printed carbon electrodes. The hybrid synthesized by the “grafting from” atom transfer radical polymerization (ATRP) process not only acted as a chiral recognition element but also provided a chiral signal amplification strategy. This occurs due to high sensitivity of conformational transition of copolymer on graphene to the weak chiral interactions that greatly facilitating the diffusion of electroactive probes and monosaccharides to the electrode surface. The described method can quantify monosaccharides, even the concentration of one enantiomer is as low as 1 nM. Apart from the demonstrated chiral distinguish ability, good selectivity toward monosaccharides in comparison to potential interference molecules was also observed. The electrodes with significant analytical performance were successfully applied for discriminating glucose enantiomers in live cells and studying their different transport mechanism. Together, the results show that the coupling of amplification-by-wettability switching concept with electrochemical method offers great promises in providing a sensitive, facile, and cost-effective solution for chiral recognition of molecules in biological process.

A Au disk nanoelectrode down to 3 nm in radius was developed by a facile and reliable method and successfully applied for monitoring dopamine release from single living vesicles. A fine etched Au wire was coated with cathodic electrophoretic paint followed by polyimide, which retracted from the tip end during curing to expose the Au nanotip. By cyclic voltammetric scanning the above tip in 0.5 M KCl, the transformation of a core-shaped apex into a geometrically well-defined Au disk nanoelectrode with different dimensions can be controllably and reproducibly achieved. Scanning electron microscopy, transmission electron microscopy, and steady-state voltammetry were used to determine the size of nanoelectrodes. The results showed that the specific etching and insulation method not only avoids the use of toxic etching solution and the uncontrollable treatment to expose the tip but also makes possible the controllable and reproducible fabrication of Au disk nanoelectrode down to 3 nm in radius. The nanoelectrodes with well-demonstrated analytical performance were further applied for amperometrically monitoring dopamine release from single rat pheochromacytoma cells with high spatial resolution.

•A new type of gold nanoparticles-sheathed glass capillary nanoelectrode was constructed.•The tip apex radius ranging from ~8.9 to ~500 nm can be prepared.•Dopamine in the striatum of anesthetic rats was successfully monitored by amperometric method.To develop in vivo monitoring strategies of neurotransmitters involved in brain chemistry is a challenging work for progress in understanding the roles that biomolecules play in pathology and physiology. Here we report a new type of gold nanoparticle-sheathed glass capillary nanoelectrode (Au/GCNE) for sensing cerebral dopamine. First, a size-controlled needle-type quartz capillary was pulled with a laser puller. Then, the capillary tip exterior was chemically functionalized with colloidal gold nanoparticles by the seed-mediated growth protocol. Through insulating the above tip with cathodic electrophoretic paint followed by heating to tune the exposed area of gold-nanoparticle-film, the Au/GCNE with tip apex radius ranging from ~8.9 to ~500 nm can be prepared. Scanning electron microscopy (SEM) and steady-state voltammetry were utilized to characterize the effective radius of nanoelectrodes. The results showed that the tip apex radius of Au/GCNE was mainly affected by the pre-pulled capillary tip, the modified AuNPs and the cathodic electrophoretic paint. By taking advantage of the modified AuNPs and the enhanced electrochemical performance of the nanoelectrode, a wide dynamic linear range from 2.0×10−8 M to 5.6×10−6 M with a low detection limit of 1.0×10−8 M (S/N=3), as well as good selectivity for dopamine, were first achieved with the Nafion-modified Au/GCNE. In addition, the designed glass substrates of Au/GCNE were mechanically stronger and their sharp tips aided in membrane penetration during implantation in the in vivo experiment. As a result, the Nafion-modified Au/GCNE was successfully applied for amperometrically monitoring dopamine in the striatum of anesthetic rats.

A novel approach for in situ synthesizing poly(ionic liquid)–Pt nanoparticle (PIL–Pt) composite in a glass capillary for fabricating filling-type electrode is reported in this work. XRD and TEM were used to characterize the as-synthesized PIL–Pt composite. Because of the modification of poly(ionic liquid)s (PILs), the PIL–Pt composite can not only be dispersed well to form a homogeneous suspension of Pt nanoparticles, but also be synthesized directly in a glass capillary with a tip radius ranging from 250 nm to 2.5 µm. By simple heating at 130 °C, the PIL–Pt composite capillary electrode was fabricated under mild conditions. With the advantages of both PILs and glass capillary, a PIL–Pt capillary electrode can provide a favourable microenvironment for the encapsulated Pt nanoparticles and promote the mass transfer rate; thus, showing a high electrocatalytic activity and stability for an oxygen reduction reaction (ORR). The present study provided a novel method for the development of high performance electrocatalysts based on the construction of PIL–Pt composite in a glass capillary for fuel cell or electrochemical sensors.