Synergistic Effect of Oxygen Vacancy-Rich SnO2 and AgCl in the Augmentation of Sustained Oxygen Reduction Reaction.

Developing a stable and methanol-tolerant electrocatalyst for a sustained oxygen reduction reaction (ORR) is of great importance for advancing direct methanol fuel cell applications. The silver-based electrocatalysts are particularly interesting among the promising non-Pt-based electrocatalysts for ORR. Herein, we report a single-step synthesis of a composite of AgCl and SnO2 with oxygen vacancy (AgCl-SnO2(VO)), which exhibits sustained and selective catalytic activity for the ORR along with excellent durability. Hydrothermal synthesis generates oxygen vacancies within the material and facilitates a strong interaction between AgCl and SnO2(VO), which effectively augments the ORR activity and the long-term stability of the composite. The composite exhibits remarkable methanol tolerance, as evidenced by a meager shift of only 0.002 V in the half-wave potential. Furthermore, the composite demonstrates excellent durability, with no noticeable changes in onset and half-wave potential even after 2500 cycles. The cost-effectiveness, durability, and ORR selectivity of this composite hold great promise toward contributing to the advancement of clean energy technology.

Electrochemical Approach for the Synthesis of Ultrasmall Cu13 Clusters and Their Application in the Detection of Endotoxin.

Ultrasmall metal nanoclusters are attractive for their size-dependent optical and electrochemical properties. Here, blue-emitting copper clusters stabilized with cetyltrimethylammonium bromide (CTAB) are synthesized using an electrochemical approach. The electrospray ionization (ESI) analysis reveals that the cluster has 13 copper atoms in the core. The clusters are then used for the electrochemical detection of endotoxin─a bacterial toxin in Gram-negative bacteria. Differential pulse voltammetry (DPV) is used to detect endotoxins with high selectivity and sensitivity. It shows a lower detection limit of 100 ag mL-1 with a linear range of 100 ag mL-1 to 10 ng mL-1. The sensor is efficient for detecting endotoxins from human blood serum samples.

Label-free detection of endotoxin and gram-negative bacteria from water using copper (I) oxide anchored reduced graphene oxide.

Bacterial contamination is a serious concern for health and environmental safety. The major toxic effect arises from the endotoxin or lipopolysaccharide (LPS) attached to the cell wall of the gram-negative bacteria. Ultrasensitive endotoxin detection is of supreme importance in sustaining food, clinical and pharmaceutical safety. Herein we report a simple electrochemical detection platform using reduced graphene oxide (rGO) combined with cuprous oxide nanoparticles for the ultrasensitive detection of LPS. The sensor uses polymyxin B (PmB) to achieve the selective response towards LPS. The sensor showed a lower detection limit (LOD) of 10 agmL-1 with linearity from 10 agmL-1 to 10 ngmL-1. Detection of LPS from whole blood is also carried out with excellent sensitivity. The sensor showed excellent recovery rates in whole blood, pointing to the capability of using the sensor in real-life clinical analysis. The sensor detects Gram-negative bacteria from sewage water with a rapid response time, indicating the effectiveness of the sensor in water quality analysis.

Cu2+-Mediated Aggregation of Gold Nanoparticles as an Optical Probe for the Detection of Endotoxin.

Endotoxins or lipopolysaccharides (LPS) present in the outer layer of Gram-negative bacteria (GNB) are responsible for bacterial toxicity. It is an environmental hazard that everyone is exposed to daily to various extents. Due to its potent toxicity, quantitative detection with very high sensitivity is essential in the food, medical, and pharmaceutical industries. Herein, we report an optical nanosensor for the rapid and sensitive detection of LPS and GNB based on the Cu2+-mediated aggregation of gold nanoparticles (Cu@AuNPs). The sensor detects LPS within a linear range of 20 ag/mL to 20 ng/mL with a lower detection limit of 0.2 ag/mL. The sensor could successfully recover spiked endotoxin in grape juice with a percentage error of ±0.2, confirming its application in the food industry. The sensor could also distinguish Gram-negative bacteria from Gram-positive bacteria, and the selectivity of the Cu@AuNP sensor toward GNB is utilized to detect Escherichia coli in wastewater. The rapid detection of E. coli without any pretreatment is a promising strategy in water analysis.

Enhancing the catalytic activity of bulk MoS2 towards hydrogen evolution reaction by the formation of MoS2-MoO3-Re2O7 heterostructure.

Two-dimensional transition metal dichalcogenides (TMD) are promising cost-effective catalysts for electrochemical/photoelectrochemical hydrogen evolution reactions (HER). One of the strategies to enhance the inherent HER activity of a catalyst is to form heterostructures. Herein, a facile thermal treatment method is reported for the synthesis of a novel heterostructure catalyst, Molybdenum disulphide-Molybdenum trioxide-Rhenium oxide (MoS2-MoO3-Re2O7), for HER. MoS2-MoO3-Re2O7 composite is prepared using bulk MoS2, which is otherwise known as a weak HER catalyst. Electrochemical and photoelectrochemical studies substantiate the enhanced catalytic activity of the MoS2-MoO3-Re2O7 composite than the MoS2 bulk towards HER. The Mott-Schottky analysis suggests the formation of p-n heterojunction, which offers large interfacial contact and facilitates easier charge separation and transfer of photogenerated charge carriers resulting in an improved photoelectrocatalytic activity. The catalyst exhibits excellent stability, confirmed by the repeated cycles of reaction.

Layer-by-Layer Assembly of Polycations and Polyanions for the Sensitive Detection of Endotoxin.

Bacterial endotoxin detection is an essential safety requisite in biomedical, food, and pharmaceutical industries. Endotoxin in a sufficient concentration on entering the human bloodstream causes detrimental effects such as septic shock, which can lead to death. Hence, the sensitive and selective detection of endotoxin also known as lipopolysaccharide (LPS) is of paramount importance. Herein, a layer-by-layer (LBL) assembly of gold-chitosan nanocomposite (CGNC)-poly(acrylic acid) (PAA)-polymyxin B (PmB) on gold (Au) electrode is employed for the sensitive and selective detection of endotoxin. The surface electric charge studies using dynamic contact mode electrostatic force microscopy (DC-EFM) revealed the successful formation of each layer on the Au electrode. The polycationic PmB is a specific bioreceptor of LPS, which binds with high affinity to the anionic groups of the carbohydrate portions of LPS molecules and facilitates the selective electrochemical detection. This surface modification method presented a sensitive and selective detection of endotoxin down to the attogram level.

Highly sensitive endotoxin detection using a gold nanoparticle loaded layered molybdenum disulfide-polyacrylic acid nanocomposite.

Endotoxins or lipopolysaccharides (LPS) are pathogens released from the outer membrane of gram-negative bacteria which produce toxic effects on humans. The sensitive and selective detection of LPS is in high demand, especially in the field of medical supplies, therapeutics and in the food industry. Herein we report a new nano-probe based on a gold nanoparticle loaded, water-soluble layered molybdenum disulfide-polyacrylic acid (Au/MoS2-PAA) nanocomposite as a label-free voltammetric aptasensor for ultrasensitive LPS detection. MoS2 nanosheets were obtained through one-step sonication assisted exfoliation of bulk MoS2 with polyacrylic acid (PAA). Au nanoparticles were incorporated into the MoS2-PAA nanocomposite and thiol terminated LPS binding aptamers (LBA) were immobilized on this. The specific binding of LPS with LBA is investigated electrochemically by differential pulse voltammetry. The apparent binding constant (Kb) of LPS with LBA has been calculated to be 1.53 × 102 mL g-1. The aptasensor demonstrated LPS detection down to the ag mL-1 level without incorporating any redox mediator and showed wide linearity from 100 ag mL-1 to 100 pg mL-1 with a low limit of detection of 29 ag mL-1. The sensor showed excellent recovery upon spiking LPS in clinical grade insulin, suggesting that LBA/Au/MoS2-PAA/GCE has promising application for the trace analysis of LPS in the field of pharmaceutical products.

Gold atomic cluster mediated electrochemical aptasensor for the detection of lipopolysaccharide.

We have constructed an aptamer immobilized gold atomic cluster mediated, ultrasensitive electrochemical biosensor (Apt/AuAC/Au) for LPS detection without any additional signal amplification strategy. The aptamer self-assemble onto the gold atomic clusters makes Apt/AuAC/Au an excellent platform for the LPS detection. Differential pulse voltammetry and EIS were used for the quantitative LPS detection. The Apt/AuAC/Au sensor offers an ultrasensitive and selective detection of LPS down to 7.94 × 10-21M level with a wide dynamic range from 0.01 attomolar to 1pM. The sensor exhibited excellent selectivity and stability. The real sample analysis was performed by spiking the diluted insulin sample with various concentration of LPS and obtained recovery within 2% error value. The sensor is found to be more sensitive than most of the literature reports. The simple and easy way of construction of this sensor provides an efficient and promising detection of an even trace amount of LPS.

Carbon nanostructures as immobilization platform for DNA: A review on current progress in electrochemical DNA sensors.

Development of a sensitive, specific and cost-effective DNA detection method is motivated by increasing demand for the early stage diagnosis of genetic diseases. Recent developments in the design and fabrication of efficient sensor platforms based on nanostructures make the highly sensitive sensors which could indicate very low detection limit to the level of few molecules, a realistic possibility. Electrochemical detection methods are widely used in DNA diagnostics as it provide simple, accurate and inexpensive platform for DNA detection. In addition, the electrochemical DNA sensors provide direct electronic signal without the use of expensive signal transduction equipment and facilitates the immobilization of single stranded DNA (ssDNA) probe sequences on a wide variety of electrode substrates. It has been found that a range of nanomaterials such as metal nanoparticles (MNPs), carbon based nanomaterials, quantum dots (QDs), magnetic nanoparticles and polymeric NPs have been introduced in the sensor design to enhance the sensing performance of electrochemical DNA sensor. In this review, we discuss recent progress in the design and fabrication of efficient electrochemical genosensors based on carbon nanostructures such as carbon nanotubes, graphene, graphene oxide and nanodiamonds.

Reduced graphene oxide-yttria nanocomposite modified electrode for enhancing the sensitivity of electrochemical genosensor.

Reduced graphene oxide-yttria nanocomposite (rGO:Y) is applied as electrochemical genosensor platform for ultrahigh sensitive detection of breast cancer 1 (BRCA1) gene for the first time. The sensor is based on the sandwich assay in which gold nanoparticle cluster labeled reporter DNA hybridize to the target DNA. Glassy carbon electrode modified with rGO-yttria serves as the immobilization platform for capture probe DNA. The sensor exhibited a fine capability of sensing BRCA1 gene with linear range of 10attomolar (aM) to 1nanomolar (nM) and a detection limit of 5.95attomolar. The minimum distinguishable response concentration is down to the attomolar level with a high sensitivity and selectivity. We demonstrated that the use of rGO:Y modified electrode along with gold nanoparticle cluster (AuNPC) label leads to the highly sensitive electrochemical detection of BRCA1 gene.

Quartz crystal microbalance genosensor for sequence specific detection of attomolar DNA targets.

A mass sensitive quartz crystal microbalance (QCM) based genosensor has been developed using breast cancer 1 (BRCA1) gene as a model gene. We modified the traditional sandwich assay by conjugating reporter probe DNA (DNA-r) with an assembly of gold nanoparticles leading to an increased mass on the surface, which enhanced the sensitivity to few orders of magnitude. The unique cleavage function of endonuclease is used for achieving the selectivity to complementary DNA over mismatched DNA. With this combination, the sensor exhibited excellent sensitivity with a detection limit of 10 aM BRCA1 gene and it showed good selectivity for even single base mismatch DNA targets. This ultrasensitive and cost-effective DNA detection protocol can be extended to the direct analysis of any non-amplified genomic DNA.

Correction: Nitrogen-fluorine co-doped titania inverse opals for enhanced solar light driven photocatalysis.

Correction for 'Nitrogen-fluorine co-doped titania inverse opals for enhanced solar light driven photocatalysis' by T. K. Rahul et al., Nanoscale, 2015, 7, 18259-18270.

Nitrogen-fluorine co-doped titania inverse opals for enhanced solar light driven photocatalysis.

Three dimensionally ordered nitrogen-fluorine (N-F) co-doped TiO2 inverse opals (IOs) were fabricated by templating with polystyrene (PS) colloidal photonic crystals (CPCs) by infiltration. During preparation, the TiO2 precursor was treated with a mixture of nitric acid and trifluoroacetic acid to facilitate N-F co-doping into the TiO2 lattice. Enhanced solar light absorption was observed in the samples as a consequence of the red shift in the electronic band gap of TiO2 due to N-F co-doping. The photonic band gap (PBG) of these TiO2 IO films was tuned by varying the sphere size of the PS CPC templates. The as-prepared N-F co-doped TiO2 IO films were used as photocatalysts for the degradation of Rhodamine B (RhB) dye under solar light irradiation. A significant enhancement in the photocatalytic activity was observed in N-F co-doped TiO2 IO films prepared using PS spheres of 215 nm as a template, with the red edge of the PBG closer to the electronic band gap (EBG) of TiO2. 100% of the dye molecules were degraded within 2 minutes under direct solar irradiation, which is one of the fastest reaction times ever reported for RhB degradation in the presence of TiO2 photocatalysts. The N-F co-doped TiO2 IO film prepared using PS of 460 nm with its PBG centered at 695 nm also showed good photocatalytic activity. It was found that the IO films displayed improved photocatalytic activity in comparison to ordinary nanocrystalline (nc)-TiO2 films. The enhancement could be attributed to the bandgap scattering effect and the slow photon effect, leading to a significant improvement in solar light harvesting.

A highly sensitive DNA sensor for attomolar detection of the BRCA1 gene: signal amplification with gold nanoparticle clusters.

Single stranded DNA fragments were conjugated onto gold nanoparticles leading to the formation of gold nanoparticle clusters upon hybridization with complementary strands. These clusters were successfully implemented for signal amplification in an electrochemical DNA sensor based on a graphene substrate. The sensor exhibited excellent sensitivity and selectivity with a detection limit of 50 attomolar target DNA.

Attomolar detection of BRCA1 gene based on gold nanoparticle assisted signal amplification.

In this work, we report a simple strategy for signal amplification using appropriately functionalized gold nanoparticles in an electrochemical genosensor which led to attomolar detection of breast cancer 1 (BRCA1) gene. The sensor was developed by the layer-by-layer assembly of mercaptopropionic acid (MPA), polyethylene glycol (PEG) functionalized gold nanoparticle (AuNPPEG), capture DNA (DNA-c), target BRCA1 DNA (DNA-t) and gold nanoparticle labeled reporter DNA (DNA-r.AuNP) on gold electrode. PEG functionalized gold nanoparticles on the MPA surface provided good electron conducting path nullifying the insulating effect of MPA and also act as a proper immobilization platform for the DNA-c by the large number of carboxyl groups present on the functionalized gold nanoparticles. We demonstrated that the incorporation of MPA functionalized gold nanoparticles (AuNPMPA) as an electrochemical label in this sensor design could significantly enhance the sensitivity in the detection. The DNA hybridization of DNA-r.AuNP with target probe was measured by chronoamperometry, electrochemical impedance spectroscopy (EIS), and scanning tunnelling spectroscopy (STS). Electrochemical quartz crystal microbalance (EQCM) experiments were used to support the detection and also to calculate the number of adsorbed molecules on the surface. Under optimum conditions the present sensor exhibited high sensitivity and a very low detection limit of 50attomolar DNA target (294.8attogram BRCA1gene/ml). It shows excellent selectivity against non complementary sequences and 3 base mismatch complementary targets. It also shows good reproducibility, stability and reusability and the developed sensor surface is suitable for point-of care applications.

Synthesis of nanotitania decorated few-layer graphene for enhanced visible light driven photocatalysis.

We report a simple method for decorating carboxyl functionalized few-layer graphene with titania (TiO2) nanoparticles by sonication and stirring under room temperature. The nanocomposites showed a remarkable improvement in visible light driven photocatalysis. From Raman and XRD analysis the number of layers of graphene was found to be 3. The TiO2 decorated few-layer graphene (FLG) sheets were characterized by electron microscopy, Raman spectroscopy, infrared spectroscopy, XRD and UV-vis spectroscopy. Titania nanoparticles were uniformly decorated on FLG matrix. The incorporation of titania on FLG enhanced the visible light photocatalytic activity of titania, lowered the electron hole recombination and improved the electron hole mobility. The enhanced life time of the charge carriers was confirmed from the photocurrent measurements. Compared to bare TiO2 nanoparticles the FLG-TiO2 nanocomposites exhibited rapid degradation of Rhodamine B (Rhd B) under solar radiation. It was found that adsorption of dye molecules and the rate of degradation have been greatly enhanced in the FLG decorated with TiO2. The rapid degradation of Rhd B using carboxyl functionalized FLG-TiO2 within 8 min under solar radiation and 20 min under 30 W UV tube with very low concentration (0.01 wt.%) of the photocatalyst is the highlight of the present report. The mechanism of degradation and charge separation ability of the nanocomposite are also explored.

Detection of glucose using immobilized bienzyme on cyclic bisureas-gold nanoparticle conjugate.

A highly sensitive electrochemical glucose sensor has been developed by the co-immobilization of glucose oxidase (GOx) and horseradish peroxidase (HRP) onto a gold electrode modified with biocompatible cyclic bisureas-gold nanoparticle conjugate (CBU-AuNP). A self-assembled monolayer of mercaptopropionic acid (MPA) and CBU-AuNP was formed on the gold electrode through a layer-by-layer assembly. This modified electrode was used for immobilization of the enzymes GOx and HRP. Both the HRP and GOx retained their catalytic activity for an extended time, as indicated by the low value of Michaelis-Menten constant. Analytical performance of the sensor was examined in terms of sensitivity, selectivity, reproducibility, lower detection limit, and stability. The developed sensor surface exhibited a limit of detection of 100nM with a linear range of 100nM to 1mM. A high sensitivity of 217.5µAmM(-1)cm(-2) at a low potential of -0.3V was obtained in this sensor design. Various kinetic parameters were calculated. The sensor was examined for its practical clinical application by estimating glucose in human blood sample.

Femtomolar level detection of BRCA1 gene using a gold nanoparticle labeled sandwich type DNA sensor.

We demonstrate the amplified detection of BRCAI gene based on the gold nanoparticle labeled DNA sensor. The sensor was based on a "sandwich" detection strategy, which involved an immobilized capture probe DNA (DNA-c), Target DNA (DNA-t) and gold nanoparticle conjugated reporter probe DNA (DNA-r.AuNP). The sensor surface was characterized by scanning electron microscopy (SEM) and scanning tunneling microscopy (STM). Detection capability of the sensor was studied with I-V measurements using either scanning tunneling microscopy (STM) or Keithley 2400 Source Meter SMU Instrument. The DNA sensor could detect up to 1 fM DNA target (5.896 fg of BRCA 1 gene/ml) and exhibited excellent selectivity against noncomplementary sequences and three base mismatch complementary targets. Good reproducibility, high sensitivity, good stability and reusability of the developed sensor surface showed its application in early cancer diagnosis.

Synthesis and characterization of gold-chitosan nanocomposite and application of resultant nanocomposite in sensors.

Chitosan gold nanocomposite (CGNC) was synthesized in a single step process and the pH dependent properties of the composite were investigated. The structure of the polymer was pH dependent and gelation of the polymer observed at pH 4-5. Transmission electron microscopy (TEM) analysis showed that the distribution of gold nanoparticles within unit area varied with the gelation of the polymer, without affecting the size of the nanoparticles. The resulting CGNC system was explored for its use as a sensor for the detection of lead. It was found that lead could be detected at a concentration of 1 µM by absorption spectroscopy. The CGNC was electrodeposited on a gold electrode and the deposited film was used for the detection of lead using cyclic voltammetry. Lead could be detected with a detection limit of 10 µM. The fabrication reproducibility for four sensor electrodes showed a relative standard deviation (RSD) of 2.09% for the determination of 1 mM lead nitrate. The developed sensor system showed high reproducibility, stability and satisfactory selectivity.

Size controlled synthesis of biocompatible gold nanoparticles and their activity in the oxidation of NADH.

Size and shape controlled synthesis remains a major bottleneck in the research on nanoparticles even after the development of different methods for their preparation. By tuning the size and shape of a nanoparticle, the intrinsic properties of the nanoparticle can be controlled leading tremendous potential applications in different fields of science and technology. We describe a facile route for the one pot synthesis of gold nanoparticles in water using monosodium glutamate as the reducing and stabilizing agent in the absence of seed particles. The particle diameter can be easily controlled by varying the pH of the reaction medium. Nanoparticles were characterized using scanning electron microscopy, UV-vis absorption spectroscopy, cyclic voltammetry, and dynamic light scattering. Zeta potential measurements were made to compare the stability of the different nanoparticles. The results suggest that lower pH favours a nucleation rate giving rise to smaller particles and higher pH favours a growth rate leading to the formation of larger particles. The synthesized nanoparticles are found to be stable and biocompatible. The nanoparticles synthesized at high pH exhibited a good electrocatalytic activity towards oxidation of nicotinamide adenine dinucleotide (NADH).