Recent Trends and Advances in Porous Metal-Organic Framework Nanostructures for the Electrochemical and Optical Sensing of Heavy Metals in Water.

With the expansion and advancement in agricultural and chemical industries, various toxic heavy metals such as lead, cadmium, mercury, zinc, copper, arsenic etc. are continuously released into the environment. Intake of sources contaminated with such toxic metals leads to various health issues. Keeping the serious effects of these toxic metal ions in view, various organic-inorganic nanomaterials based sensors have been exploited for their detection via optical, electrochemical and colorimetric approaches. Since a chemical sensor works on the principle of interaction between the sensing layer and the analytes, a sensor material with large surface area is required to enable the largest possible interaction with the target molecules and hence the sensitivity of the chemical sensor. However, commonly employed materials such as metal oxides and conducting polymers tend to feature relatively low surface areas, and hence resulting in low sensitivity of the sensor. Metal-Organic Frameworks (MOFs) nanostructures are another category of organic-inorganic materials endowed with large surface area, ultra-high and tunable porosity, post-synthesis modification features, readily available active sites, catalytic activity, and chemical/thermal stability. These properties provide high sensitivity to the MOF based sensors due to the adsorption of large number of target analytes. The current review article focuses on MOFs based optical and electrochemical sensors for the detection of heavy metals.

Advancements in 2D Materials Based Biosensors for Oxidative Stress Biomarkers.

The imbalance in generation of reactive oxygen species and its depletion causes oxidative stress. Because of its importance, there is a need to explore the role of oxidative stress biomarkers. The limitations of the conventional methods cause the researchers look for other alternatives. Biosensors are highly promising candidates for the detection of trace quantities for various analytes with high specificity, selectivity, and quick response time. Nanomaterial based matrices are the most popular choice while fabricating a biosensor. Two-dimensional (2D) materials such as graphene, transitional metal dichalcogenides, and various metal oxides have been used for the biosensing of different oxidative stress analytes. High electron mobility, good optical properties, tunable properties, high yields, easy synthesis, and availability make these materials the first choice. In this review, we have comprehensively discussed various biomarkers associated with oxidative stress. The review will provide information related to different kinds and various synthesis procedures employed for 2D nanomaterials. The major focus of the review is to elaborate the role of 2D nanomaterial based structures for the optical and electrochemical methods for the detection of oxidative stress. This review is an effort to help the researchers better understand various 2D based transducers available for the detection of oxidative stress biomarkers.

Sensitive impedimetric detection of troponin I with metal-organic framework composite electrode.

Metal-organic frameworks (MOFs) are promising materials for biosensing applications due to their large surface to volume ratio, easy assembly as thin films, and better biocompatibility than other nanomaterials. Their application in electrochemical biosensing devices can be realized by integrating them with other conducting materials, like polyaniline (PANI). In the present research, a composite of a copper-MOF (i.e., Cu3(BTC)2) with PANI has been explored to develop an impedimetric sensor for cardiac marker troponin I (cTnI). The solvothermally synthesized Cu3(BTC)2/PANI composite has been coated as a thin layer on the screen-printed carbon electrodes (SPE). This electroconductive thin film was conjugated with anti-cTnI antibodies. The above formed immunosensor has allowed the impedimetric detection of cTnI antigen over a clinically important concentration range of 1-400 ng mL-1. The whole process of antigen analysis could be completed within 5 min. The detection method was specific to cTnI even in the co-presence of other possibly interfering proteins.

Advancement in biosensors for inflammatory biomarkers of SARS-CoV-2 during 2019-2020.

COVID-19 pandemic has affected everyone throughout the world and has resulted in the loss of lives of many souls. Due to the restless efforts of the researchers working hard day and night, some success has been gained for the detection of virus. As on date, the traditional polymerized chain reactions (PCR), lateral flow devices (LFID) and enzyme linked immunosorbent assays (ELISA) are being adapted for the detection of this deadly virus. However, a more exciting avenue is the detection of certain biomarkers associated with this viral infection which can be done by simply re-purposing our existing infrastructure. SARS-CoV-2 viral infection triggers various inflammatory, biochemical and hematological biomarkers. Because of the infection route that the virus follows, it causes significant inflammatory response. As a result, various inflammatory markers have been reported to be closely associated with this infection such as C-reactive proteins, interleukin-6, procalcitonin and ferritin. Sensing of these biomarkers can simultaneously help in understanding the illness level of the affected patient. Also, by monitoring these biomarkers, we can predict the viral infections in those patients who have low SARS-CoV-2 RNA and hence are missed by traditional tests. This can give more targets to the researchers and scientists, working in the area of drug development and provide better prognosis. In this review, we propose to highlight the conventional as well as the non-conventional methods for the detection of these inflammatory biomarkers which can act as a single platform of knowledge for the researchers and scientists working for the treatment of COVID-19.

White graphene quantum dots as electrochemical sensing platform for ferritin.

The use of hexagonal boron nitride quantum dots (hBN QDs) as an electrochemical sensor for ferritin is reported for the first time. These QDs were synthesized using a simple liquid exfoliation method. The synthesized material was characterized using analytical techniques such as UV-visible, Fourier transform infrared (FTIR) and Raman spectroscopy, X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HR-TEM) to study different aspects of the QDs. These QDs were explored for their plausible application as a platform for the electrochemical detection of ferritin. For this, electrochemical impedance spectroscopy was used as a sensing technique and disposable hBN QD functionalized screen printed electrodes were used as a sensing platform. The developed immunosensor had a dynamic linear range from 10-2000 ng mL-1 of ferritin concentration with a limit of detection of 1.306 ng mL-1. The immunosensor was highly selective, did not deviate in the presence of interfering agents and was also highly reproducible.

Chemoresistive Room-Temperature Sensing of Ammonia Using Zeolite Imidazole Framework and Reduced Graphene Oxide (ZIF-67/rGO) Composite.

The present work demonstrates the application of a composite of the zeolite imidazole framework (ZIF-67) and reduced graphene oxide (rGO), synthesized via a simple hydrothermal route for the sensitive sensing of ammonia. The successful synthesis of ZIF-67 and rGO composite was confirmed with structural and spectroscopic characterizations. A porous structure and a high surface area (1080 m2 g-1) of the composite indicate its suitability as a gas sensing material. The composite material was coated as a thin film onto interdigitated gold electrodes. The sensor displays a change in its chemoresistive property (i.e., resistance) in the presence of ammonia (NH3) gas. A sensor response of 1.22 ± 0.02 [standard deviation (sd)] is measured for 20 ppm of NH3, while it shows a value of 4.77 ± 0.15 (sd) for 50 ppm of NH3. The fabricated sensor is reproducible and offers a stable response, while also providing tolerance against humidity and some other volatile compounds. The average response and recovery times of the sensor, for 50 ppm NH3 concentration, are found to be 46.5 ± 2.12 (sd) and 66.5 ± 2.12 (sd) s, respectively. The limit of detection of the sensor was found to be 74 ppb.

Highly Sensitive Optical Detection of Escherichia coli Using Terbium-Based Metal-Organic Framework.

Metal-organic frameworks (MOFs) are envisaged as highly useful for the development of biosensors. Herein, for the first time, we report the optical detection of Escherichia coli using a water-dispersible terbium MOF (Tb-BTC; BTC, 1,3,5-benzenetricarboxylic acid). The successful synthesis of Tb-BTC is verified using spectroscopic and morphological techniques like UV-vis, fluorescence and FTIR spectroscopy, X-ray diffraction analysis, and electron microscopy. Tb-BTC has been bio-interfaced with anti-E. coli antibodies and then investigated as a biosensor for E. coli. The biosensor displays detection ability in an analyte concentration range of 1.3 × 102 to 1.3 × 108 cfu/mL with a detection limit of 3 cfu/mL, having a response time of 5 min and a total analysis time of about 20-25 min. The results are also found to be reproducible and specific in the presence of some other interfering bacterial species. As demonstrated, the present sensor provides highly sensitive and specific detection of E. coli in fruit juice sample. To the best of our knowledge, this is the first report to showcase the potential of the MOF-based fluorescent biosensor for the detection of E. coli.

Microfluidic-Based Electrochemical Immunosensing of Ferritin.

Ferritin is a clinically important biomarker which reflects the state of iron in the body and is directly involved with anemia. Current methods available for ferritin estimation are generally not portable or they do not provide a fast response. To combat these issues, an attempt was made for lab-on-a-chip-based electrochemical detection of ferritin, developed with an integrated electrochemically active screen-printed electrode (SPE), combining nanotechnology, microfluidics, and electrochemistry. The SPE surface was modified with amine-functionalized graphene oxide to facilitate the binding of ferritin antibodies on the electrode surface. The functionalized SPE was embedded in the microfluidic flow cell with a simple magnetic clamping mechanism to allow continuous electrochemical detection of ferritin. Ferritin detection was accomplished via cyclic voltammetry with a dynamic linear range from 7.81 to 500 ng·mL-1 and an LOD of 0.413 ng·mL-1. The sensor performance was verified with spiked human serum samples. Furthermore, the sensor was validated by comparing its response with the response of the conventional ELISA method. The current method of microfluidic flow cell-based electrochemical ferritin detection demonstrated promising sensitivity and selectivity. This confirmed the plausibility of using the reported technique in point-of-care testing applications at a much faster rate than conventional techniques.

Design and development of an optical reflective notch filter using the ion assisted deposition technique with stepwise modulated thickness for avionics applications.

This paper presents research work about the design and fabrication of a 44-layer optical reflective notch filter. The performance of the fabricated notch filter was studied at normal (0°) and oblique (45°) incidence angle. In addition, the paper also discusses a three-layer broadband antireflective coating on both sides of the multilayer stack to suppress the ripples in the passband region. The thickness-modulated reflective stack of the filter was designed by using the materials Al2O3 (1.63) and SiO2 (1.46). Optimization of the multilayer stack was carried out by using the damped least-squares algorithm. The theoretical and experimental results from the ion-assisted e-beam deposited samples for single notch reflective filters are presented. Good agreement in the design and experimental results was observed when the deposition process was controlled by time of evaporation. Further, the filter was characterized for the optical properties by using a UV-VIS-NIR spectrophotometer, surface morphology and protective properties using field emission scanning electron microscopy, a coherence correlation interferometer, and water contact angle.

Lysine-Functionalized Tungsten Disulfide Quantum Dots as Artificial Enzyme Mimics for Oxidative Stress Biomarker Sensing.

The color generating from the biochemical reaction between 3,3',5,5'-tetramethylbenzidine and Lysine@WS2 QDs was used a signal for the detection of hydrogen peroxide. The QDs were prepared using a combination of techniques, that is, probe sonication and hydrothermal treatment. Analysis via UV-vis spectroscopy, Fourier transform infrared and Raman spectroscopy, X-ray diffraction, energy-dispersive spectroscopy, and transmission electron microscopy yielded detailed information on the nature and characteristics of these quantum dots. Furthermore, as-synthesized quantum dots were studied for their capability to mimic peroxidase enzyme using 3,3',5,5'-tetramethylbenzidine as a substrate. Consequently, a colorimetric sensor utilizing Lysine@WS2 QDs could detect hydrogen peroxide in a range of 0.1-60 µM with a response time of 5 min. The same material was used for H2O2 detection using impedance spectroscopy, which yielded a dynamic range of 0.1-350 µM with a response time of 30-40 s.

Label-free approach for electrochemical ferritin sensing using biosurfactant stabilized tungsten disulfide quantum dots.

A novel approach for the synthesis of biosurfactant stabilized/functionalized tungsten disulfide (WS2-B) quantum dots (QDs) and its application for ferritin immunosensor is reported. These 2-D layered material derived quantum dots are synthesized via one-step liquid exfoliation method and biosurfactant was used as a functionalization and stability providing moiety. The biosurfactant provided a clean and green method for both the synthesis and in-situ functionalization of the QDs. Exhaustive characterization using analytical techniques was done to study various aspects of the synthesized quantum dots. The functionalized quantum dots (WS2-B QDs) were further explored for their possible application as an electroactive platform. For this, the working area of commercially available screen-printed electrodes (SPE) was functionalized with these WS2-B QDs to construct a sensor platform. This sensor platform was then used for fabrication of ferritin immunosensor, using ferritin specific antibodies. Cyclic Voltammetry (CV) and Differential Pulse Voltammetry (DPV) techniques were used for electrochemical immunosensing of ferritin. Though, the achieved linear range for ferritin detection (10-1500 ng mL-1) is same with both the techniques but regression coefficient and limit of detection are better in differential pulse voltammetry. The limit of detection was found to be 3.800 ng mL-1 in DPV and 6.048 ng mL-1 in CV. The immunosensor is highly selective, reproducible and is stable for about 60 days.

A graphene electrode functionalized with aminoterephthalic acid for impedimetric immunosensing of Escherichia coli.

A screen-printed electrode prepared from graphene oxide (GO) has been functionalized with 2-aminoterephthalic acid, followed by the exploitation of this functional material in an electrochemical immunoassay for Escherichia coli (E. coli) by immobilizing the antibody on its surface. The functionalization steps followed a straightforward approach and were proven by various instrumental techniques. The detection of E. coli with antibody immobilized electrodes was performed using electrochemical impedance spectroscopy. The analyses were carried out using the hexacyanoferrate redox couple as the electrochemical probe. The present method has a wide analytical range (from 2.2 × 102 to 2.2 × 108 cfu.mL-1), a low limit of detection (2 cfu.mL-1), fast response (4 min), and good stability (up to 2 months). The analytical performance of the biosensor was comparable to the previously reported electrochemical biosensors for E. coli. As such, the approach of functionalization of graphene with 2-aminoterephthalic acid should be useful to allow the development of other similar sensing systems for other environmentally and clinically important analytes. Graphical abstractSchematic representation of the preparation and the function of an amino-functionalized graphene oxide (NH2-GO) based impedimetric biosensor for the electrochemical detection of E. coli.

Development of an advanced electrochemical biosensing platform for E. coli using hybrid metal-organic framework/polyaniline composite.

Because of numerous merits (e.g., the possibility of their synthesis in 1-D, 2-D, and 3-D forms, large surface-to-volume ratio, and flexible framework functionality), metal-organic frameworks (MOFs) are envisaged as excellent media for the development of biosensors for diverse analytes present in environmental media. The present research work, for the first time, reports the development of a Cu-MOF based electrochemical biosensor for highly sensitive detection of E. coli bacteria. In order to realize an MOF-based electrochemically active platform, Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylic acid) was mixed with polyaniline (PANI). The spectroscopic/morphological characterizations of the resulting composite were established with the aid of FT-IR, UV-visible spectroscopy, X-ray diffraction, electron microscopy, and surface area analysis. The thin films of Cu3(BTC)2-PANI, on an indium-tin oxide (ITO) substrate, were bio-interfaced with anti-E. coli antibodies for use as a novel biosensing electrode. Based on the electrochemical impedance spectroscopy (EIS) technique of signal measurement, the above sensor exhibited high sensitivity to detect very low concentrations of E. coli (2cfu/mL) in a short response time (~2 min) and was also selective in the presence of other non-specific bacteria. As a novel highlight of the research, this new MOF/PANI based detection platform for E. coli has shown improved performance than many of the previously reported electrochemical biosensors.

A three-phase copper MOF-graphene-polyaniline composite for effective sensing of ammonia.

In this work, a three-phase composite material consisting of SiO2-coated Cu-MOF, single layer graphene, and aniline was synthesized. In the presence of ammonium persulfate as an oxidant, the aniline component of this mixture was polymerized to polyaniline to bridge Cu-MOF and graphene in the composite. Hence, a new sensory material with a highly porous nature (MOF) and superior conduction properties (graphene/polyaniline) was constructed. More specifically, the inclusion of Cu-MOF in the matrix facilitated the acquisition of an electrochemically active sensory material with a high surface area of about 756 m2/g. The potential application of this porous semiconducting material was demonstrated for sensitive detection of ammonia in a linear detection range over 1-100 ppm with a low limit of detection of 0.6 ppm.

A New Electrolytic Synthesis Method for Few-Layered MoS2 Nanosheets and Their Robust Biointerfacing with Reduced Antibodies.

We report an efficient method for the synthesis of few-layered MoS2 nanosheets and demonstrate their application in the label-free detection of the prostate-specific antigen (PSA) cancer marker. As a novel strategy, the electro-dissolution of molybdenum metal sheets in the presence of Na(+) and S(2-) ions led to the formation of Na(+) intercalated MoS2. Further exfoliation by ultrasonication yielded the desired formation of few-layered MoS2 nanosheets. After comprehensive characterization, the synthesized MoS2 nanosheets were channeled in a field-effect transistor (FET) microdevice. Chemically reduced anti-PSA antibodies were immobilized on the MoS2 channel above the FET microdevice to construct a specific PSA immunosensor. The antibodies were deliberately reduced to expose the hinge-region disulfide bonds. This approach offered a robust and site-directed immunosensing device through biointerfacing of the sulfhydryl groups (-SH) in the reduced antibody with the surface S atoms of MoS2. This device was validated as an effective immunosensor with a low detection limit (10(-5) ng/mL) over a wide linear detection range (10(-5) to 75 ng/mL).

A facile chemical route for recovery of high quality zinc oxide nanoparticles from spent alkaline batteries.

Recycling of spent domestic batteries has gained a great environmental significance. In the present research, we propose a new and simple technique for the recovery of high-purity zinc oxide nanoparticles from the electrode waste of spent alkaline Zn-MnO2 batteries. The electrode material was collected by the manual dismantling and mixed with 5M HCl for reaction with a phosphine oxide reagent Cyanex 923® at 250°C for 30min. The desired ZnO nanoparticles were restored from the Zn-Cyanex 923 complex through an ethanolic precipitation step. The recovered particle product with about 5nm diameter exhibited fluorescent properties (emission peak at 400nm) when excited by UV radiation (excitation energy of 300nm). Thus, the proposed technique offered a simple and efficient route for recovering high purity ZnO nanoparticles from spent alkaline batteries.

Formation of High-Purity Indium Oxide Nanoparticles and Their Application to Sensitive Detection of Ammonia.

High-purity In2O3 nanoparticles were recovered from scrap indium tin oxide substrates in a stepwise process involving acidic leaching, liquid-liquid extraction with a phosphine oxide extractant, and combustion of the organic phase. The morphological and structural parameters of the recovered nanoparticles were investigated to support the formation of the desired products. These In2O3 nanoparticles were used for sensitive sensing of ammonia gas using a four-probe electrode device. The proposed sensor offered very quick response time (around 10 s) and highly sensitive detection of ammonia (at a detection limit of 1 ppm).

Immunosensing of Atrazine with Antibody-Functionalized Cu-MOF Conducting Thin Films.

This work reports the assembly of thin films of a silica (SiO2)-modified copper-metal organic framework, Cu3(BTC)2 [Cu3(BTC)2@SiO2, BTC = benzene-1,3,5-tricarboxylic acid] on a conducting substrate of NH2-BDC [NH2-BDC = 2-aminobenzene-1,4-dicarboxylic acid] doped polyaniline (PANI). Assembled Cu3(BTC)2@SiO2/BDC-PANI thin films displayed electrical conductivity in the range of 35 µA. These thin films were conjugated with antiatrazine antibodies to create a novel immunosensing platform. Various structural and spectral characteristics of the synthesized material and its bioconjugate were investigated. The developed immunosensor was used for the conductometric sensing of atrazine. The detection of atrazine was achieved with a high sensor sensitivity (limit of detection = 0.01 nM) and specificity in the presence of diverse pesticides (e.g., endosulfan, parathion, paraoxon, malathion, and monochrotophos).

Phosphinic acid functionalized carbon nanotubes for sensitive and selective sensing of chromium(VI).

Single-walled carbon nanotubes (SWCNTs) have been functionalized with a phosphinic acid derivative 'bis(2,4,4-trimethylpentyl) phosphinic acid' (PA/d). It has been achieved by treating the chlorinated SWCNTs with PA/d at 80°C. Successful functionalization and different nanomaterial properties have been investigated by UV-vis-NIR, FTIR, Raman spectroscopy, AFM and FE-SEM. PA/d conjugated SWCNTs (CNT-PA) are dispersible in some common organic solvents, e.g. CH2Cl2, DMF, CHCl3, and THF. The 'CNT-PA' complex was spin-casted on boron doped silicon wafer. Thus fabricated sensing electrode is demonstrated for sensitive and selective electrochemical sensing of chromium(VI) ions. A linear response is obtained over a wide range of Cr(VI) concentration (0.01-10 ppb). The sensor's sensitivity and the limit of detection are observed to be 35 ± 4 nA/ppb and 0.01 ppb, respectively. The practical utility of the proposed sensor is demonstrated by determining the Cr(VI) concentration in an industrial effluent sample and an underground water sample.