One-Pot Synthesis of Nanoflower-Like Zn2SnS4 as Nanozymes for Highly Sensitive Electrochemical Detection of H2O2 Released by Living Cells.

The sensitive and reliable nanozyme-based sensor enables the detection of low concentrations of H2O2 in biological microenvironments, it has potential applications as an in-situ monitoring platform for cellular H2O2 release. The uniformly dispersed bimetallic sulfide (Zn2SnS4) nanoflowers were synthesized via a one-pot hydrothermal method and the two kinds of metal ions can serve as morphology and structure directing agents for each other in the synthetic process. The nanoparticles were utilized as nanozyme materials to fabricate a novel electrochemical sensor, and it exhibits a distinct electrochemical response towards H2O2 with excellent stability and detection capability (with a minimum detection limit of 1.79 nM (S/N=3)), the excellent characteristics facilitate the precise detection of low concentrations of H2O2 in biological microenvironments. Use the macrophages differentiated from leukemia THP-1 cells as a representative sensing model, the sensor was successfully utilized for real-time monitoring of the release of H2O2 induced by living cells, which has significant potential applications in clinical diagnosis and cancer treatment.

Competitive substitution in europium metal-organic gel for signal-on electrochemiluminescence detection of dipicolinic acid.

Metal-organic gels (MOGs) emerged as an attractive luminescent soft material for electrochemiluminescence (ECL). In this work, a cathodic ECL-activated europium metal-organic gel (Eu-MOG) has been synthesized by a facile mixing of Eu3+ with 4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine (Hcptpy) under mild conditions. The prepared Eu-MOG is highly mesoporous for co-reactant permeation to produce an ultra-stable and high-efficient ECL, based on the antenna effect of Eu3+ coordinating with Hcptpy. Moreover, dipicolinic acid (DPA) can competitively coordinate with Eu3+ instead of water molecules, producing an enhanced ECL signal. Therefore, an ECL enhancement assay was developed for DPA detection. There was a linear relationship between the ECL intensity and the logarithmic concentration of DPA in the 0.01-1 µM range, and the detection limit is 7.35 nM. This work displays the promising application of Eu-MOG in the ECL field, opening a broad inspection for seeking a new generation of ECL luminophores.

Metal-enhanced fluorometric formaldehyde assay based on the use of in-situ grown silver nanoparticles on silica-encapsulated carbon dots.

Fluorescent nanoparticles were prepared by encapsulating carbon dots (CDs) within silica spheres and then modifying these spheres with amino groups (CD@SiO2-NH2). On the basis of the silver mirror reaction, Ag+ assembled on the surface of CD@SiO2-NH2 is reduced to silver nanoparticles (AgNPs) by formaldehyde. The in-situ grown AgNPs cause a visually distinguishable fluorescence enhancement. This metal-enhanced effect was investigated by transmission electron microscopy and spectroscopic characterization, and the relevant conditions were optimized. CD@SiO2-NH2-Ag+ fluorescent probes were loaded onto nano-sponge pieces for the analysis of formaldehyde gas. The blue fluorescence emission (peaking at 466 nm) in response to formaldehyde is greatly enhanced (up to 5.2 times) over other species. There is a linear relationship between the fluorescence enhancement and formaldehyde gas concentration in the range of 10 ppb to 1 ppm, and the detection limit is 3 ppb. The fluorimetric assay needs 30 min for the reaction, and the fluorescent nano-sponge pieces are disposable. Graphical abstractSchematic representation of the metal-enhanced fluorescence (MEF) induced by in-situ grown silver nanoparticles on silica-encapsulated carbon dots, and its application in formaldehyde gas assays.

Homogeneous Electrochemiluminescence Biosensor for the Detection of RNase A Activity and Its Inhibitor.

Ribonuclease A (RNase A) is increasingly considered as a biomarker for tumor diagnosis, and it is of great significance to develop an ultrasensitive, cost-effective assay for RNase A detection. Electrochemiluminescence (ECL) technology has distinctive advantages in the development of biosensors for diverse targets. However, most of the ECL biosensors require the complex process of electrode modification, which is laborious and time consuming. In this work, an immobilization-free homogeneous ECL assay was developed for the highly sensitive detection of RNase A activity for the first time. On the basis of the fact that RNase A can specifically hydrolyze RNA at the site of ribonucleotide uracil (rU), a rU-containing chimeric DNA probe is designed and labeled with Ru(bpy)32+ (act as ECL indicator). The chimeric DNA probe hardly diffuses to the surface of negatively charged indium tin oxide (ITO) electrode due to the strong electrostatic repulsion between the negatively charged DNA and ITO electrode, resulting in a weak ECL signal detected. When the RNase A is present, the chimeric DNA probe is hydrolyzed into small fragments, which contains little negative charge and can diffuse easily to the ITO electrode surface due to the decreased electrostatic repulsion. In this case, an enhanced ECL signal can be detected. Under the optimal conditions, there is a linear relationship between the ECL signal and the concentration of RNase A in the range of 0.001-0.10 ng/mL, and the detection limit is 0.2 pg/mL. In addition, the proposed ECL sensing system is also applied to detect the RNase A inhibitor, taking As3+ as an example. The proposed homogeneous ECL sensing system provides a new approach for the highly sensitive and convenient detection of RNase A as well as other ribonucleases only by redesigning a responding chimeric DNA probe.

Cationic Carbon Dots for Modification-Free Detection of Hyaluronidase via an Electrostatic-Controlled Ratiometric Fluorescence Assay.

Carbon dots (CDs) emerge as excellent fluorescent nanomaterials, but the full exploitation and application of their exceptional properties in the development of fluorescence assay are still rare. In this work, cationic carbon dots (C-CDs) covered with plenty of positive charges on the surface were synthesized through a facile ultrasonic method. Negatively charged hyaluronic acid (HA) caused the aggregation of positively charged C-CDs and neutral red (NR) along its linear chain via electrostatic adsorption, leading to a remarkable Förster resonance energy transfer (FRET) from C-CDs to NR. However, the presence of hyaluronidase (HAase) resulted in the enzymolysis of HA, as well as the liberation of C-CDs and NR. The corresponding change of fluorescence color from red to green-yellow afforded a reliable ratiometric assay for HAase. Also the ratio of fluorescence intensity for C-CDs (I525) to that for NR (I630) was used for quantitative detection of HAase. The proposed sensing system was easily operated in aqueous media with a detection limit of 0.05 U/mL. This strategy provides a new approach for the wider application of some special CDs in detecting biomolecules.

Low voltage-driven, high-performance TiO2 thin film transistors with MHz switching speed.

High-speed circuits based on thin film transistors (TFTs) show promising potential applications in biomedical imaging and human-machine interactions. One of the critical requirements for high-speed electronic devices lies in high-frequency switching or amplification at low voltages, typically driven by batteries (∼3.0 V). To date, however, most electrical performances of metal oxide TFTs are measured under direct current (DC) conditions, and their dynamic switching behaviour is scarcely explored and studied systematically. Here in this work, we present low voltage-driven, high-performance TiO2 thin film transistors, which can be operated at a switching speed of MHz. Our proposed TiO2 TFTs demonstrated a high on-off ratio of 107, together with a subthreshold swing (SS) of ∼150 mV Dec-1 averaged over four orders of magnitude, which can be further reduced below 100 mV Dec-1 when the temperature cools to 77 K. Additionally, the TiO2 TFTs exhibit excellent gate-pulse switching at various frequencies ranging from 1.0 Hz to 1.0 MHz. We also explored the potential application of the TiO2 TFTs as logic gates by constructing a resistive-loaded inverter, which shows stable operation at 10 kHz frequency and various temperatures. Thus, our results show the great potential of TiO2 TFTs as a new platform for high-speed electronic applications.

Electrophoretic deposition of Ru(bpy)32+ in vertically-ordered silica nanochannels: A solid-state electrochemiluminescence sensor for prolidase assay.

Prolidase (PLD) plays a crucial role as a dipeptidase in various physiological processes, specifically involved in the cleavage of proline-containing dipeptides for efficient recycling of proline. The accurate determination of PLD activity holds significant importance in clinical diagnosis. Herein, a solid-state electrochemiluminescence (ECL) biosensor was developed to address the urgent need for PLD assay. The Ru(bpy)32+ was electrophoretically deposited within the nanochannels of vertically-ordered mesoporous silica film (VMSF) on indium tin oxide (ITO) electrodes. The Ru(bpy)32+-deposited VMSF/ITO (Ru-VMSF/ITO) exhibited a remarkable ECL response towards proline, attributed to the enhanced concentration of the reactants and improved electron transfer resulting from the nanoconfinement effect. As PLD specifically enzymolyzed the Gly-Pro dipeptide to release proline, a proline-mediated biosensor was developed for PLD assay. Increased PLD activity led to enhanced release of proline into the porous solid-state ECL sensors, resulting in a more robust ECL signal. There was a linear relationship between ΔECL intensity and logarithmic concentration of PLD in the range of 10-10000 U/L, with a detection limit of 1.98 U/L. Practical tests demonstrated the reliability and convenience of the proposed bioassay, making it suitable for widespread application in PLD assays.

Carbon dots and chitosan composite film based biosensor for the sensitive and selective determination of dopamine.

A simple, sensitive and reliable dopamine (DA) biosensor was developed based on a carbon dots (CDs) and chitosan (CS) composite film modified glassy carbon electrode (CDs-CS/GCE). Under optimal conditions, the CDs-CS/GCE showed a better electrochemical response for the detection of DA than that of the glassy carbon electrode (GCE). The oxidation peak current (Ipa) of DA was linear with the concentration of DA in the range from 0.1 µM to 30.0 µM with the limit of detection as 11.2 nM (3S/N). The CDs-CS/GCE was applied to the detection of DA content in an injection solution of DA with satisfactory results.

A planar and uncharged copper(II)-picolinic acid chelate: Its intercalation to duplex DNA by experimental and theoretical studies and electrochemical sensing application.

Using an external redox-active molecule as a DNA hybridization indicator is still a popular strategy in electrochemical DNA biosensors because it is label-free and the multi-site binding can enhance the response signal. A planar and uncharged transition metal complex, Cu(PA)2 (PA = picolinic acid) with excellent electrochemical activity has been synthesized and its interaction with double-stranded DNA (dsDNA) is studied by experimental electrochemical methods and theoretical molecular docking technology. The experimental results reveal that the copper complex interacts with dsDNA via specific intercalation, which is verified by the molecular docking result. The surface-based voltammetric analysis demonstrates that the planar Cu(PA)2 can effectively accumulate within the electrode-confined hybridized duplex DNA rather than the single-stranded probe DNA. Based on this phenomenon, the Cu(PA)2 is utilized as an electrochemical hybridization indicator for the detection of oligonucleotides. The sensing assays show that upon incubation in Cu(PA)2 solution, the probe electrode does not display any Faraday signal, but the hybridized one has a pair of strong redox peaks corresponding to the electrochemistry of Cu(PA)2, showing excellent hybridization indicating function of Cu(PA)2 without background interference. The signal intensity of Cu(PA)2 is dependent on the concentrations of the target oligonucleotide ranging from 1 fM to 100 nM with an experimental detection limit of 1.0 fM. Due to the specific intercalation of Cu(PA)2 with dsDNA, the biosensor also exhibits good ability to recognize oligonucleotide with different base mismatching degree.

Homogeneous and label-free electrochemiluminescence aptasensor based on the difference of electrostatic interaction and exonuclease-assisted target recycling amplification.

The difference of electrostatic interaction between free Ru(phen)32+ and Ru(phen)32+ embedded in double strand DNA (dsDNA) to the negatively charged indium tin oxide (ITO) electrode has been applied to develop a homogeneous and label-free electrochemiluminescence (ECL) aptasensor for the first time. Ochratoxin A (OTA) has been chosen as the model target. The OTA aptamer is first hybridized with its complementary single strand DNA (ssDNA) to form dsDNA and then interacted with Ru(phen)32+ via the grooves binding mode to form dsDNA-Ru(phen)32+ complex, which remains negatively charged feature as well as low diffusion capacity to the negatively charged ITO electrode surface owing to the electrostatic repulsion. Meanwhile, the intercalated Ru(phen)32+ in the grooves of dsDNA works as an ECL signal reporter instead of the labor-intensive labeling steps and can generate much more ECL signal than that from the labeling probe. In the presence of target, the aptamer prefers to form an aptamer-target complex in lieu of dsDNA, which induces the releasing of Ru(phen)32+ from the dsDNA-Ru(phen)32+ complex into the solution. With the assistance of RecJf exonuclease (a ssDNA specific exonuclease), the released ssDNA and the aptamer in the target-complex were digested into mononucleotides. In the meantime, the target can be also liberated from OTA-aptamer complex and induce target cycling and large amount of free Ru(phen)32+ present in the solution. Since Ru(phen)32+ contains positive charges, which can diffuses easily to the ITO electrode surface because of electrostatic attraction, causing an obviously enhanced ECL signal detected. Under the optimal conditions, the enhanced ECL of the system has a linear relationship with the OTA concentration in the range of 0.01-1.0 ng/mL with a detection limit of 2 pg/mL. This innovative system not only expands the immobilization-free sensors in the electrochemiluminescent fields, but also can be developed for the detection of different targets easily with the same strategy by changing the aptamer used.

Facile construction of a highly sensitive DNA biosensor by in-situ assembly of electro-active tags on hairpin-structured probe fragment.

An ultrasensitive DNA biosensor has been developed through in-situ labeling of electroactive melamine-Cu(2+) complex (Mel-Cu(2+)) on the end of hairpin-like probe using gold nanoparticles (AuNPs) as the signal amplification platform. The 3'-thiolated hairpin-like probe was first immobilized to the gold electrode surface by the Au-S bond. The AuNPs were then tethered on the free 5'-end of the immobilized probe via the special affinity between Au and the modified -NH2. Followed by, the Mel and Cu(2+) were assembled on the AuNPs surface through Au-N bond and Cu(2+)-N bond, respectively. Due to the surface area and electrocatalytic effects of the AuNPs, the loading amount and electron transfer kinetic of the Mel-Cu(2+) were enhanced greatly, resulting in significantly enhanced electrochemical response of the developed biosensor. Compared with the synthesis process of conventional electroactive probe DNA accomplished by homogeneous method, the method presented in this work is more reagent- and time-saving. The proposed biosensor showed high selectivity, wide linear range and low detection limit. This novel strategy could also be extended to the other bioanalysis platforms such as immunosensors and aptasensors.

Graphene Oxide Directed One-Step Synthesis of Flowerlike Graphene@HKUST-1 for Enzyme-Free Detection of Hydrogen Peroxide in Biological Samples.

A novel metal-organic framework (MOF)-based electroactive nanocomposite containing graphene fragments and HKUST-1 was synthesized via a facile one-step solvothermal method using graphene oxide (GO), benzene-1,3,5-tricarboxylic acid (BTC), and copper nitrate (Cu(NO3)2) as the raw materials. The morphology and structure characterization revealed that the GO could induce the transformation of HKUST-1 from octahedral structure to the hierarchical flower shape as an effective structure-directing agent. Also, it is interesting to find out that the GO was torn into small fragments to participate in the formation of HKUST-1 and then transformed into the reduction form during the solvothermal reaction process, which dramatically increased the surface area, electronic conductivity, and redox-activity of the material. Electrochemical assays showed that the synergy of graphene and HKUST-1 in the nanocomposite leaded to high electrocatalysis, fast response, and excellent selectivity toward the reduction of hydrogen peroxide (H2O2). Based on these remarkable advantages, satisfactory results were obtained when the nanocomposite was used as a sensing material for electrochemical determination of H2O2 in the complex biological samples such as human serum and living Raw 264.7 cell fluids.

Immobilization free electrochemical biosensor for folate receptor in cancer cells based on terminal protection.

The determination of folate receptor (FR) that over expressed in vast quantity of cancerous cells frequently is significant for the clinical diagnosis and treatment of cancers. Many DNA-based electrochemical biosensors have been developed for FR detection with high selectivity and sensitivity, but most of them need complicated immobilization of DNA on the electrode surface firstly, which is tedious and therefore results in the poor reproducibility. In this study, a simple, sensitive, and selective electrochemical FR biosensor in cancer cells has been proposed, which combines the advantages of the convenient immobilization-free homogeneous indium tin oxide (ITO)-based electrochemical detection strategy and the high selectivity of the terminal protection of small molecule linked DNA. The small molecule of folic acid (FA) and an electroactive molecule of ferrocence (Fc) were tethered to 3'- and 5'-end of an arbitrary single-stranded DNA (ssDNA), respectively, forming the FA-ssDNA-Fc complex. In the absence of the target FR, the FA-ssDNA-Fc was degraded by exonuclease I (Exo I) from 3'-end and produced a free Fc, diffusing freely to the ITO electrode surface and resulting in strong electrochemical signal. When the target FR was present, the FA-ssDNA-Fc was bound to FR through specific interaction with FA anchored at the 3'-end, effectively protecting the ssDNA strand from hydrolysis by Exo I. The FR-FA-ssDNA-Fc could not diffuse easily to the negatively charged ITO electrode surface due to the electrostatic repulsion between the DNA strand and the negatively charged ITO electrode, so electrochemical signal reduced. The decreased electrochemical signal has a linear relationship with the logarithm of FR concentration in range of 10fM to 10nM with a detection limit of 3.8fM (S/N=3). The proposed biosensor has been applied to detect FR in HeLa cancer cells, and the decreased electrochemical signal has a linear relationship with the logarithm of cell concentration ranging from 100-10000cell/mL. Compared with the traditional heterogeneous electrochemical FR biosensors, the proposed biosensor owns the merits of the simplicity and high specificity, presenting the great potential application in the area of early diagnosis of cancers.

Low-background, highly sensitive DNA biosensor by using an electrically neutral cobalt(II) complex as the redox hybridization indicator.

An electrically neutral cobalt complex, [Co(GA)2(phen)] (GA = glycollic acid, phen = 1,10-phenathroline), was synthesized and its interactions with double-stranded DNA (dsDNA) were studied by using electrochemical methods on a glassy carbon electrode (GCE). We found that [Co(GA)2(phen)] could intercalate into the DNA duplex through the planar phen ligand with a high binding constant of 6.2(±0.2)×10(5) M(-1). Surface studies showed that the cobalt complex could electrochemically accumulate within the modified dsDNA layer, rather than within the single-stranded DNA (ssDNA) layer. Based on this feature, the complex was applied as a redox-active hybridization indicator to detect 18-base oligonucleotides from the CaMV35S promoter gene. This biosensor presented a very low background signal during hybridization detection and could realize the detection over a wide kinetic range from 1.0×10(-14) M to 1.0×10(-8) M, with a low detection limit of 2.0 fM towards the target sequences. The hybridization selectivity experiments further revealed that the complementary sequence, the one-base-mismatched sequence, and the non-complementary sequence could be well-distinguished by the cobalt-complex-based biosensor.

A sensitive DNA biosensor based on a facile sulfamide coupling reaction for capture probe immobilization.

A novel DNA biosensor was fabricated through a facile sulfamide coupling reaction. First, the versatile sulfonic dye molecule of 1-amino-2-naphthol-4-sulfonate (AN-SO3(-)) was electrodeposited on the surface of a glassy carbon electrode (GCE) to form a steady and ordered AN-SO3(-) layer. Then the amino-terminated capture probe was covalently grafted to the surface of SO3(-)-AN deposited GCE through the sulfamide coupling reaction between the amino groups in the probe DNA and the sulfonic groups in the AN-SO3(-). The step-by-step modification process was characterized by electrochemistry and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. Using Ru(NH3)6(3+) as probe, the probe density and the hybridization efficiency of the biosensor were determined to be 3.18×10(13) strands cm(-2) and 86.5%, respectively. The hybridization performance of the biosensor was examined by differential pulse voltammetry using Co(phen)3(3+/2+) (phen=1,10-phenanthroline) as the indicator. The selectivity experiments showed that the biosensor presented distinguishable response after hybridization with the three-base mismatched, non-complementary and complementary sequences. Under the optimal conditions, the oxidation peak currents of Co(phen)3(3+/2+) increased linearly with the logarithm values of the concentration of the complementary sequences in the range from 1.0×10(-13)M to 1.0×10(-8)M with a regression coefficient of 0.9961. The detection limit was estimated to be 7.2×10(-14)M based on 3σ.

Thorough removal of inorganic and organic mercury from aqueous solutions by adsorption on Lemna minor powder.

The adsorption ability of duckweed (Lemna minor) powders for removing inorganic and organic mercury (methyl and ethyl mercury) has been studied using cold vapour atomic absorption spectrometry. The optimal adsorption conditions were: (a) the pH value of the solution 7.0 for inorganic and ethyl mercury, 9.0 for methyl mercury, and (b) equilibrium adsorption time 10, 20, and 40 min for inorganic mercury, methyl mercury, and ethyl mercury, respectively. After adsorption by L. minor powder for 40 min, when the initial concentrations of inorganic and organic mercury were under 12.0 µg L(-1) and 50.0 µg L(-1), respectively, the residual concentrations of mercury could meet the criterion of drinking water (1.0 µg L(-1)) and the permitted discharge limit of wastewater (10.0 µg L(-1)) set by China and USEPA, respectively. Thorough removal of both inorganic and organic mercury from aqueous solutions was reported for the first time. The significant adsorption sites were C-O-P and phosphate groups by the surface electrostatic interactions with aqueous inorganic and organic mercury cations, and then the selective adsorption was resulted from the strong chelating interaction between amine groups and mercury on the surface of L. minor cells.