Rare Earth-Carbon Dots Nanocomposite-Modified Glass Nanopipettes: Electro-Optical Detection of Bacterial ppGpp.

As an important alarmone nucleotide, guanosine 3'-diphosphate-5'-diphosphate (ppGpp) can regulate the survival of bacteria under strict environmental conditions. Direct detection of ppGpp in bacteria with high sensitivity and selectivity is crucial for elucidating the role of ppGpp in bacterial stringent response. Herein, the terbium-carbon dots nanocomposite (CDs-Tb) modified glass nanopipet was developed for the recognition of ppGpp. The CDs-Tb in glass nanopipette preserved their fluorescence properties as well as the coordination capacity of Tb3+ toward ppGpp. The addition of ppGpp not only led to the fluorescence response of CDs-Tb but also triggered variations of surface charge inside the glass nanopipet, resulting in the ionic current response. Compared with nucleotides with similar structures, this method displayed good selectivity toward ppGpp. Moreover, the dual signals (fluorescence and ionic current) offered a built-in correction for potential interference. Apart from the high selectivity, the proposed method can determine the concentration of ppGpp from 10-13 to 10-7 M. Taking advantage of the significant analytical performance, we monitored ppGpp in Escherichia coli under different nutritional conditions and studied the relationship between ppGpp and DNA repair, which is helpful for overcoming antibiotic resistance and promoting the development of potential drugs for antibacterial treatment.

In Vivo Electrochemical Biosensors: Recent Advances in Molecular Design, Electrode Materials, and Electrochemical Devices.

Electrochemical biosensors provide powerful tools for dissecting the dynamically changing neurochemical signals in the living brain, which contribute to the insight into the physiological and pathological processes of the brain, due to their high spatial and temporal resolutions. Recent advances in the integration of in vivo electrochemical sensors with cross-disciplinary advances have reinvigorated the development of in vivo sensors with even better performance. In this Review, we summarize the recent advances in molecular design, electrode materials, and electrochemical devices for in vivo electrochemical sensors from molecular to macroscopic dimensions, highlighting the methods to obtain high performance for fulfilling the requirements for determination in the complex brain through flexible and smart design of molecules, materials, and devices. Also, we look forward to the development of next-generation in vivo electrochemical biosensors.

Stimuli-responsive polymers for interface engineering toward enhanced electrochemical analysis of neurochemicals.

Neurochemical monitoring can provide important insights into the chemical communications in the brain and neurological diseases. Although electrochemical sensors have promoted the development of neurochemical analysis, the limited analytical performance of the existing sensors restrict our understanding of the roles that chemical signals play in the brain. The central nervous system is composed of a large number of neurochemical species. Meanwhile, it is difficult to monitor neurochemicals with high sensitivity because of the kinetic barrier of mass transport and electron/ion transfer. More importantly, to fabricate a "smart" electrochemical sensor for neurochemicals, the engineering of an electrode surface with switchable properties and a response is urgently needed. This review focuses on the construction and application of electrochemical sensors based on stimuli-responsive polymers. The response of polymers to external stimuli can not only enhance the target recognition, but also modulate the electrochemical signals, thus providing smart electrochemical sensing platform with improved analytical performance, including high selectivity, sensitivity, and controllability. In this review, we first introduce the design strategy of bio-responsive stimuli-responsive polymers and highlight the relationship between their structure and molecular recognition efficiency. Then, we summarize the electrochemical techniques with different sensing principles and emphasize the contribution that stimuli-responsive polymers made to the conversion of chemical/electrochemical reactions into electric signals. Finally, the opportunities and limitations of stimuli-responsive polymer-modified electrochemical sensors for neurochemical analysis will be discussed. Taking advantage of the development of novel materials, electrochemical techniques and microelectronic engineering, the advanced devices (e.g., antifouling, flexible, miniaturized, and multi-functional) with remarkable analytical performance will benefit the evaluation of neurochemicals, which can promote a deep understanding of brain events and the diagnosis and treatment of neurological diseases.

Rational design of a self-assembled surfactant film in nanopipettes: combined fluorescence and electrochemical sensing.

Herein, a generalizable method based on the formation of a self-assembled surfactant film was reported to build a nanopipette system. Using this nanopipette, it was found that arginine metabolism shows an age-related difference in Alzheimer's disease.

A highly selective ATP-responsive biomimetic nanochannel based on smart copolymer.

ATP-sensitive potassium (KATP) channels couple intracellular metabolism to the electrical activity by regulating K+ flux across the plasma membrane, thus playing an important role in both normal and pathophysiology. To understand the mechanism of ATP regulating biological ion channels, developing an ATP-responsive artificial nanochannel is an appealing but challenging topic because KATP channel is a heteromultimer of two subunits (potassium channel subunit (Kir6.x) and sulfonylurea receptor (SUR)) and exhibit dynamic functions with adjustability and reversibility. Inspired by the structure of KATP channels, we designed a smart copolymer modified nanochannel that may address the challenge. In the tricomponent poly(N-isopropylacrylamide) (PNIPAAm, PNI)-based copolymer system, phenylthiourea was used to bind the phosphate units of nucleotides and phenylboronic acid was introduced to combine the pentose ring of the nucleoside unit. Besides, a -COOH group with electron-withdrawing property was added into the phenylthiourea units, which may promote the hydrogen-bond-donating ability of thiourea. Specially, the smart copolymer not only provided static binding sites for recognition but also translated the recognition of ATP into their dynamic conformational transitions by changing the hydrogen-bonding environments surrounding PNIPAAm chains, thus achieving the gating function of nanochannel, which resembled the integration and coordination of Kir6.x and SUR units in active KATP. The ATP-regulated ion channel exhibited excellent stability and reversibility. This study is the first example showing how to learn from nature to assemble the ATP-responsive artificial nanochannel and demonstrate the possible mechanism of ATP gating.

A ratiometric electrochemical sensor for selectively monitoring monoamine oxidase A in the live brain.

Herein, an electrochemical method for selectively sensing and accurately quantifying monoamine oxidase A (MAO-A) in the cortex and thalamus of a live mouse brain was reported. Using this tool, it was found that MAO-A increased Ca2+ entry into neurons via the TPRM2 channel in the live mouse brain of an AD model.

Coordination of Ligand-Protected Metal Nanoclusters and Glass Nanopipettes: Conversion of a Liquid-Phase Fluorometric Assay into an Enhanced Nanopore Analysis.

We propose a unique concept for transforming the liquid-phase fluorometric assay into an enhanced nanopore analysis, which is based on the analyte binding-mediated changes in the surface chemistry of noble metal nanostructures in a confined space. In a proof-of-concept trial, the bovine serum albumin-protected gold nanoclusters (BSA-Au NCs) were designed as the sensing unit for biothiol determination. Through the specific interaction between biothiols and BSA-Au NCs, the validation system not only performed well in aqueous fluorescent detection but also can be developed into a more selective and sensitive nanopore sensor. In the confined space of the nanopore, the BSA-Au NC film with high density formed, and the addition of biothiols triggered the fluorescence enhancement as well as the ionic current response, hence leading to the construction of the dual-signal-output (fluorescence/ion current signal) system. The fluorescence signal proved that the ionic current change corresponded to the specific recognition process, improving the reliability of our nanopore method. Moreover, the ionic current response from the BSA-Au NC film can be used to quantify cysteine in a broad dynamic range of 0.001-1 pM with a detection limit as low as 1 fM. Such a strategy can be used to detect biothiols in complex biological fluids such as human serum. Therefore, the present work provided a new design strategy for a glass nanopipette sensor inspired by the principles of numerous and diverse fluorometric assays. It also sheds light on how the coupling of electrical and optical signals improves the accuracy, sensitivity, and selectivity of the glass nanopipette platform.

In situ detection of hydroxyl radicals in mitochondrial oxidative stress with a nanopipette electrode.

A novel nanopipette electrode was facilely constructed for the detection of ˙OH based on the inner surface wettability. The nanopipettes with excellent analytical performance were empolyed for in situ detection of ˙OH changes induced by mitochondrial oxidative stress and further utilized to study the association of ˙OH with Alzheimer's disease.

A Robust Au-C≡C Functionalized Surface: Toward Real-Time Mapping and Accurate Quantification of Fe2+ in the Brains of Live AD Mouse Models.

Described here is that Au-C≡C bonds showed the highest stability under biological conditions, with abundant thiols, and the best electrochemical performance compared to Au-S and Au-Se bonds. The new finding was also confirmed by theorical calculations. Based on this finding, a specific molecule for recognition of Fe2+ was designed and synthesized, and used to create a selective and accurate electrochemical sensor for the quantification of Fe2+ . The present ratiometric strategy demonstrates high spatial resolution for real-time tracking of Fe2+ in a dynamic range of 0.2-120 µM. Finally, a microelectrode array with good biocompatibility was applied in imaging and biosensing of Fe2+ in the different regions of live mouse brains. Using this tool, it was discovered that the uptake of extracellular Fe2+ into the cortex and striatum was largely mediated by cyclic adenosine monophosphate (cAMP) through the CREB-related pathway in the brain of a mouse with Alzheimer's disease.

Interface engineering of microelectrodes toward ultrasensitive monitoring of ß-amyloid peptides in cerebrospinal fluid in Alzheimer's disease.

Ultrasensitive detection of monomeric ß-amyloid peptides is of fundamental significance for studying the pathological progression of Alzheimer's disease (AD). In this article, by facilely engineering a gold microelectrode interface, we developed a novel electrochemical biosensor for sensitive and selective monitoring of ß-amyloid peptide (Aß) monomers in cerebrospinal fluid (CSF). Through specific Cu2+-Aß-hemin coordination, Aß directed the assembly of Cu2+-PEI/AuNPs-hemin nanoprobes into network aggregates on a microelectrode interface, which promoted the enrichment of Aß monomers on the microelectrode. Furthermore, the AuNP aggregate promotes the deposition of silver nanoparticles, which were utilized for the electrochemical stripping analysis of the Aß monomer. The proposed method displayed ultra-sensitivity for Aß monomers with the detection limit down to 0.2 pM. Besides, high selectivity toward Aß monomers was observed. These remarkable analytical performances render the electrochemical biosensor useful for evaluating the dynamic change of Aß monomer level in CSF of live mice with AD, promoting the investigation of the role that Aß monomers play in brain events.

Sensitive and Selective Measurement of Hydroxyl Radicals at Subcellular Level with Tungsten Nanoelectrodes.

Hydroxyl radical (•OH) is an essential reactive oxygen species involved in critical cell functions. However, the mechanisms controlling its subcellular localization and intracellular level during health and disease remain poorly understood. This is due to the challenge of detecting •OH that are highly reactive and consequently short-lived (in vivo half-life of ∼10-9 s). Herein, we present tungsten nanoelectrodes functionalized with stable 1-hexanethiol (HAT) for selective and sensitive detection of •OH at the subcellular level via the destruction of the self-assembled monolayer of HAT on the nanoelectrode tip. Taking advantage of the ultrasmall nanotip and the super mechanical toughness, the tungsten nanoelectrode could easily penetrate a single living cell without inducing any observable damage. Controlled by a high precision micromanipulator, the •OH level in RAW 264.7 murine macrophages under amyloid ß (Aß) induced oxidative stress were first investigated by the nanoelectrodes at the subcellular level. Moreover, the results revealed the cordycepin-mediated cytoprotection of macrophages through modulation of PI3K/Akt pathway activity and introduction of heme oxygenase-1 (HO-1). We believe that the developed nanoelectrochemical method has shown great capacities for the study of potential drugs for therapeutic intervention of Alzheimer's disease.

Rational Design of Stimuli-Responsive Polymers Modified Nanopores for Selective and Sensitive Determination of Salivary Glucose.

The great pain and stress from finger-prick glucose measurements have resulted in great motivation to find noninvasive glucose monitoring technologies where salivary glucose measurement is desirable. However, the relative low concentration of glucose and coexisting chemicals in saliva challenges the sensitive and selective salivary glucose detection. In this article, we have rationally designed and constructed a salivary glucose sensor by modifying the inner wall of the Au-decorated glass nanopore with stimuli-responsive copolymer poly(3-(acryloylthioureido) phenylboronic acid-co-N-isopropylacrylamide) (denoted as PATPBA-co-PNIPAAm) via Au-S interaction. Notably, upon recognition of glucose, the copolymer could undergo a wettability switch and pKa shifts in the boronic acid functional groups, which significantly regulated the ion transport through nanopores, thus showing improved sensitivity with the detection limit of 1 nM. Moreover, benefiting from the multivalent boronic acid-glucose interaction and the cooperation of thiourea units, the copolymer exhibited good selectivity for glucose detection against the coexisting saccharides and other biological molecules in saliva. The nanopores with well-demonstrated analytical performance were finally applied for monitoring glucose in saliva. Together, this work unveiled a new platform for glucose detection in saliva, and promised to provide a new strategy for detecting other biomolecules in accessible biofluid involved in physiological and pathological events.

Enantiomers of Single Chirality Nanotube as Chiral Recognition Interface for Enhanced Electrochemical Chiral Analysis.

Although separation of single-walled carbon nanotubes (SWCNTs) according to their helicity and handedness has been attracting tremendous interest recently, exploration of the left- and right-handed SWCNT enantiomers (defined as "M" and "P") to chiral sensing still remains in the early stage. Here we presented a new electrochemical sensor for chiral discrimination, which for the first time amplified the chiral selection on the electrode surface based on the left- or right-handed semiconducting SWCNT enantiomers with (6,5)-enriched chirality. The enantioselectivity was demonstrated by different peak current response to analyte enantiomers, observed in differential pulse voltammogram (DPV). Chiral distinguishing might be a result of the formation of an efficient chiral nanospace originating from the high purity of single enantiomer of (6,5) SWCNT. The obtained chiral electrodes were also applied to determine the enantiomeric excess (ee) of DOPA. There was a good linear relationship between DPV peak currents and % ee of l-DOPA. This study is the first example showing how the structure of chiral SWCNTs influences electrochemical chiral recognition.

Rational Design of a Stimuli-Responsive Polymer Electrode Interface Coupled with in Vivo Microdialysis for Measurement of Sialic Acid in Live Mouse Brain in Alzheimer's Disease.

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.

Biomimetic Mineralization of Gold Nanoclusters as Multifunctional Thin Films for Glass Nanopore Modification, Characterization, and Sensing.

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.

Single Biosensor for Simultaneous Quantification of Glucose and pH in a Rat Brain of Diabetic Model Using Both Current and Potential Outputs.

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.

Ionic liquids modified graphene oxide composites: a high efficient adsorbent for phthalates from aqueous solution.

In 2015, more than 30% of erasers were found to contain a PAE content that exceeded the 0.1% limit established by the Quality and Technology Supervision Bureau of Jiangsu Province in China. Thus, strengthening the supervision and regulation of the PAE content in foods and supplies, in particular, remains necessary. Graphene oxide (GO) and its composites have drawn great interests as promising adsorbents for polar and nonpolar compounds. However, GO-based adsorbents are typically restricted by the difficult separation after treatment because of the high pressure in filtration and low density in centrifugation. Herein, a series of novel ionic liquids modified graphene oxide composites (GO-ILs) were prepared as adsorbents for phthalates (PAEs) in eraser samples, which overcame the conventional drawbacks. These novel composites have a combination of the high surface area of graphene oxide and the tunability of the ionic liquids. It is expected that the GO-ILs composites can be used as efficient adsorbents for PAEs from aqueous solution. This work also demonstrated a new technique for GO-based materials applied in sample preparation.

An efficient core-shell fluorescent silica nanoprobe for ratiometric fluorescence detection of pH in living cells.

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.

Functional surface engineering of quantum dot hydrogels for selective fluorescence imaging of extracellular lactate release.

Selective and sensitive detection of extracellular lactate is of fundamental significance for studying the metabolic alterations in tumor progression. Here we report the rational design and synthesis of a quantum-dot-hydrogel-based fluorescent probe for biosensing and bioimaging the extracellular lactate. By surface engineering the destabilized quantum dot sol with Nile Blue, the destabilized Nile-Blue-functionalized quantum dot sol cannot only self-assemble forming quantum dot hydrogel but also monitor lactate in the presence of nicotinamide adenine dinucleotide cofactor and lactate dehydrogenase through fluorescence resonance energy transfer. Notably, the surface engineered quantum dot hydrogel show high selectivity toward lactate over common metal ions, amino acids and other small molecules that widely coexist in biological system. Moreover, the destabilized Nile-Blue-functionalized quantum dots can encapsulate isolated cancer cells when self-assembled into a hydrogel and thus specifically detect and image the extracellular lactate metabolism. By virtue of these properties, the functionalized quantum dot hydrogel was further successfully applied to monitor the effect of metabolic agents.

In vivo monitoring of local pH values in a live rat brain based on the design of a specific electroactive molecule for H(+).

pH plays an important role in the biochemical, ion-regulatory, or electrical machinery of nerve and glial cells, and is considered to be related to a number of degenerative diseases. Herein, we first develop a two-channel electrochemical ratiometric biosensor for local pH determination in a live rat brain, and report the accurate pH values in the different regions of live brains upon global cerebral ischemia.