Popularity of graphene oxide (GO) became ballistic in the last few years – in short, it almost seems that now everyone wants to work with graphene. However, as the number of electrochemical papers about graphene grows, so is the amount of misinterpretations. Our work was mostly focused on preparation of GO and its electrochemical properties, namely catalysis. Our results show, that chemical preparation of GO by strong oxidation is very tricky. Several approaches were tested, such as different sizes of graphite particles, oxidation mixtures, even repetitive preparations under the same conditions usually resulted in different products. Another big topic are impurities which are present due to natural occurrence in graphite and some of them might be also introduced during purification processes of GO. This could also lead to often observed catalytic activity of graphene. From our observations neither selectivity nor significant electrocatalytic abilities of the reduced graphene oxide modified electrodes towards H2O2 were observed. Moreover, our results suggest that GO does not need linking molecules in order to be deposited onto the surface of the electrodes, since it attaches spontaneously and keeps its electrochemical properties. Graphene is usually reported as a superior material, but its proper description and characterization should not be underestimated.

About graphene oxide

Environmental pollution has been a known threat to our world due to the rapid urbanization, changing lifestyle of people, and modern industrialization. Therefore, there is an urgent need to develop novel sensing approaches having promising performance with high reliability and sensitivity for the precise monitoring of various pollutants. Metal–organic frameworks (MOFs) have been intensively investigated by many researchers as electrode modifiers for electrochemical sensing due to their excellent properties and efficiency. Diverse MOF-based electrochemical sensing systems are applied for environmental analysis for the sensitive, rapid and cost-effective determination of various analytes because of their unique structures, and properties, including the tunable pore size, high surface area, high catalytic activity, and high density of active sites. The aim of this review article is to evaluate the application of MOFs in the electrochemical sensing of environmental pollutants including heavy metal ions, pesticides, phenolic compounds, nitroaromatic compounds, antibiotics, nitrite, and hydrazine. Current limitations and future directions for the application of MOF-based electrochemical sensors for the detection of environmental pollutants are discussed.

Performance of metal-organic frameworks in the electrochemical sensing of environmental pollutants

In the current water monitoring panorama, certain benefits are expected to arise when the modus operandum of analysis shifts from sampling-based to purely in-situ approaches. Since the appearance of the first submersible conductivity-temperature-depth (CTD) probe in the 80 s, clear efforts to move towards decentralized strategies have been reported in the literature, with some having even been brought to the stage of fruitful commercialization. Among the portfolio of available analytical techniques, only a handful of approaches offer clear potential for implementation in submersible devices, in terms of adequate analytical features, autonomy, feasibility of miniaturization, and ease of portability. Electrochemical sensors have demonstrated excellent characteristics for this purpose, particularly in the detection of ions. The present review analyses electrochemical sensors that have shown an aptitude for in-situ measurements of ions, including trace metals, nutrients, and carbon species in seawater. The previous 5 years have been selected as the main period for review, although in some instances comment is made upon earlier contributions to the field or commercialized devices, where these are deemed to exemplify crucial technological advancements. There is a notable lack of electrochemical sensors being deployed in in-situ applications, and this scarcity is even more stark when seawater is considered: only a very few cases have been demonstrated under such challenging conditions. With the aim of providing inspiration towards genuine advances in the field of in-situ seawater analysis, this work also highlights some laboratory scale research as well as studies concerning other environmental waters than seawater.

Electrochemical sensors for in-situ measurement of ions in seawater

Ratiometric determination endows fluorescence (FL) sensors improving accuracy and sensitivity by measuring intensity ratio of well-resolved emission peaks. Herein, tricolor-emission carbon dots (T-CDs) have been developed for ratiometric recognition of Al3+ and pyrophosphate ion (PPi). T-CDs were fabricated from hematoxylin and Tris by one-pot hydrothermal treatment. As-prepared T-CDs engender amusing triple emission bands, including bright yellow and orange FL, but weak blue FL. Upon introducing Al3+, yellow and orange FL is distinctly quenched while blue one is enhanced. On subsequent addition of PPi, T-CDs’ FL can be partially recovered. The ratiometric fluorescent changes of T-CDs have been utilized to detect Al3+ and PPi in aqueous solution with linear ranges of 5–15 μM and 0.75–10 μM, respectively, as well as limits of detection of 0.8 nM and 2.2 nM, respectively. More importantly, laser scanning confocal microscope imaging of HeLa cells demonstrates that T-CDs could be applied to visually monitor Al3+ and PPi fluctuations in living cells, illustrating that proposed T-CDs have tremendous potential in biological systems.

Tricolor emission carbon dots for label-free ratiometric fluorescent and colorimetric recognition of Al3+ and pyrophosphate ion and cellular imaging

A simple optical aptasensor has been synthesized for the detection of calcium ions. This sensing approach employs a semiconductor quantum dot (QD) – gold nanoparticle as the donor – quencher pair and operates on the principle of fluorescence resonant energy transfer (FRET). On binding with calcium ions, the DNA aptamer undergoes a conformational change, which changes the distance between the quantum dot and the gold nanoparticle, conjugated on the 5’ terminal and 3’ terminal of the aptamer respectively. This phenomenon results in the quenching of the quantum dot emission. In this sensor, a maximum quenching of 22.42 ± 0.71 % has been achieved at 35 nM calcium ion concentration while the limit of detection has been determined to be 3.77 pM. The sensor has been found to have high specificity for calcium ions in comparison to other metal ions like sodium, magnesium and potassium. The molecular aptabeacons also demonstrated successful endocytosis and FRET based calcium ion detection in osteocyte cells when conjugated with a cell penetrating peptide (DSS).

Fluorescence Resonant Energy Transfer-Based Quantum Dot Sensor for the Detection of Calcium Ions

In recent years, non-toxic quantum dot has caught the attention of biomedical fields. However, the inherent cytotoxicity of QDs makes its biomedical application painful, and is a major drawback of this method. In this paper, a non-toxic and water-soluble quantum dot AgInZnS-GO using graphene oxide was synthesized. A simple model of state complex was also established, which is produced through a combination of quantum dots and protein. The interaction between AIZS-GO QDs and human serum albumin (HSA) has significant meaning in vivo biological application. Herein, the binding of AIZS-GO QDs and HSA were researched using fluorescence spectra, Uv-visible absorption spectra, FT-IR spectra, and circular dichroism (CD) spectra. The results of fluorescence spectra demonstrate that AIZS-GO QDs have an obvious fluorescence quenching effect on HSA. The quenching mechanism is static quenching, which implies that some type of complex was produced by the binding of QDs and HSA. These results were further proved by Uv-visible absorption spectroscopy. The Stern-Volmer quenching constant Ksv at various temperatures (298 K, 303 K, 308 K) were acquired from analyzing Stern-Volmer plots of the fluorescence quenching information. The Van't Hoff equation could describe the thermodynamic parameters, which demonstrated that the van der Waals and hydrogen bonds had an essential effect on the interaction. FT-IR spectra and CD spectra further indicate that AIZS-GO QDs can alter the structure of HSA. These spectral methods show that the quantum dot can combine well with HSA. The experimental results showed that AgInZn-GO water-soluble quantum dots have good biocompatibility, which can be combined with proteins to form new compounds which have no cytotoxicity and biological practicability. It provides an important basis for the combination of quantum dots and specific proteins as well as fluorescent labeling.

/pdf/study-on-aginzns-graphene-oxide-non-toxic-quantum-dots-for-55zgloixz7.pdf

Study on AgInZnS-graphene oxide non-toxic quantum dots for biomedical sensing

Nitrite is a toxic substance, when excessive nitrite enters the human body, it will be seriously harmful to human. At present, the detection methods of nitrite are complicated to operate and require expensive detection instruments. Therefore, an effective, fast and highly selective nanogold film interdigital electrode sensors that can detect nitrite easily and quickly is developed in the work. Firstly, the variation of the sensitivity of nanogold film nitrite sensors with concentrations (1 mol/L, 10-1 mol/L, 10-2 mol/L, 10-3 mol/L, 10-4 mol/L, and 10-5 mol/L) was measured by experiments. Then, Chrome-black T was modified to the surface of the nanogold film interdigital electrodes by electrochemical polymerization, and the film of chrome-black T had affinity for nitrite ions, so nitrite ions were enriched on the sensor surface. The change law of the impedance signal of the modified nanogold film nitrite sensors after being added to different concentrations of sodium nitrite solution were also concluded. The study demonstrates that the larger the concentration of sodium nitrite solution is added to the modified interdigital electrodes, the smaller impedance and resistance of the modified interdigital electrodes are reflected. Finally, specificity of the modified interdigital electrode sensors has been demonstrated. The novel interdigital electrode sensors can detect the concentration of nitrite solution conveniently and quickly with only 30 s. Therefore, the prospect of applying the novel nanogold film interdigital electrode sensors to the detection of nitrite in blood, body fluid, food and drinking water is promising.

/pdf/a-fast-and-highly-selective-nitrite-sensor-based-on-4zwifxeh07.pdf

A Fast and Highly Selective Nitrite Sensor Based on Interdigital Electrodes Modified With Nanogold Film and Chrome-Black T.

DOI: 10.1002/elps.202100168 The cover picture shows how affinity-based electroanalytical biosensors can be enhanced by integrated AC electrokinetic (ACEK) effects. ACEK effects, including AC electrothermal (ACET) effect, AC electroosmosis (ACEO) effect and dielectrophoretic (DEP) effect, are produced by applying AC signals to the interdigitated electrode arrays. ACEK effects induce directed movement of microflows and particles (e.g., proteins, nucleic acids, bacteria, small molecules, etc.) in the solution, so that the analytes in the solution can be quickly routed toward the surface of biosensors, thus improving the binding efficiency between the analytes and the probes on the surface of the sensor. Studies have shown that the method holds great promise for achieving high sensitivity, low detection limit and rapid turnaround. 

Front Cover: Enhancing affinity‐based electroanalytical biosensors by integrated AC electrokinetic enrichment—A mini review

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Study on AgInZnS-graphene oxide non-toxic quantum dots for biomedical sensing

A Fast and Highly Selective Nitrite Sensor Based on Interdigital Electrodes Modified With Nanogold Film and Chrome-Black T.

Front Cover: Enhancing affinity‐based electroanalytical biosensors by integrated AC electrokinetic enrichment—A mini review