A universal green synthesis approach of several quantum dots (QDs), including iron oxide (Fe 3 O 4 ) QDs, gold (Au) QDs, zirconium oxide (ZrO 2 ) QDs, and graphene (Gr) QDs, was demonstrated in this study. Nanozyme-based signal amplification strategy of those QDs was also examined, which may have significant advantages for the development of biosensors with visual read-outs. A synergetic peroxidase-like activity of Fe 3 O 4 QDs was observed for the facile, visual, fieldportable, and sensitive detection of H 2 O 2 . The fluorescence emission, UV−vis spectrum, and circular dichroism spectra of the Fe 3 O 4 QDs were located at 419, 395, and 335 nm, respectively. The band gap energy of synthesized Fe 3 O 4 QDs was estimated to be 2.0 eV based on the Tauc plot. The X-ray photoelectron spectroscopic analysis revealed that the Fe(II) to Fe(III) ratio was increased when the Fe 3 O 4 NPs were converted to the Fe 3 O 4 QDs. The nonenzymatic activity of those QDs was further investigated using a mixture of 3,3′,5,5′-tetramethylbenzidine (TMB) and H 2 O 2 . The Fe 3 O 4 QDs possessed high peroxidaselike activity and exhibited Michaelis−Menten kinetics behavior. Kinetic studies revealed that the Fe 3 O 4 QDs demonstrated a higher affinity toward TMB than the standard enzyme horseradish peroxidase. This prolonged peroxidase-like Fe 3 O 4 QD catalytic activity was successfully applied for the detection of H 2 O 2 with a limit of detection (LOD) of 3.87 nM, which was calculated based on the standard deviation method. While similar approach was examined with Au QDs, ZrO 2 QDs, and Gr QDs in this study, no characteristic enzymatic activity was observed, which confirmed the unique properties of the Fe 3 O 4 QDs. The facile synthesis approach and the unique nanozymatic activity of the Fe 3 O 4 QDs described in the present study open a new horizon in materials chemistry and the development of colorimetric biosensors for environmental, energy, and medical applications.

Neurotransmitters are molecules that transfer chemical signals between neurons to convey messages for any action conducted by the nervous system. All neurotransmitters are medically important; the detection and analysis of these molecules play vital roles in the diagnosis and treatment of diseases. Among analytical strategies, electrochemical techniques have been identified as simple, inexpensive, and less time-consuming processes. Electrochemical analysis is based on the redox behaviors of neurotransmitters, as well as their metabolites. A variety of electrochemical techniques are available for the detection of biomolecules. However, the development of a sensing platform with high sensitivity and selectivity is challenging, and it has been found to be a bottleneck step in the analysis of neurotransmitters. Nanomaterials-based sensor platforms are fascinating for researchers because of their ability to perform the electrochemical analysis of neurotransmitters due to their improved detection efficacy, and they have been widely reported on for their sensitive detection of epinephrine, dopamine, serotonin, glutamate, acetylcholine, nitric oxide, and purines. The advancement of electroanalytical technologies and the innovation of functional nanomaterials have been assisting greatly in in vivo and in vitro analyses of neurotransmitters, especially for point-of-care clinical applications. In this review, firstly, we focus on the most commonly employed electrochemical analysis techniques, in conjunction with their working principles and abilities for the detection of neurotransmitters. Subsequently, we concentrate on the fabrication and development of nanomaterials-based electrochemical sensors and their advantages over other detection techniques. Finally, we address the challenges and the future outlook in the development of electrochemical sensors for the efficient detection of neurotransmitters.

Hydrogen production via electrochemical water splitting is limited thermodynamically by the sluggish oxygen evolution reaction (OER) at the anode. The use of noble metal-based catalysts leads to an economic bottleneck because of the high cost associated with such materials. This article is an electrochemical investigation of an economically viable and advanced OER catalyst made of cobalt/graphene nanocomposite quantum dots (QDs). A series of characterization techniques, such as high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and fluorescence measurements, were performed, and they confirmed the formation of graphene QDs as well as the formation of cobalt-based QDs. A very high current density of 43.16 mA cm–2 was observed for the QD nanocomposite, whereas smaller current densities were seen for Co nanoparticles (21.1 mA cm–2) and a benchmark Pt/C commercial catalyst (5.99 mA cm–2). Furthermore, an overpotential of only 0.49 V is required for the composite material at 10 mA cm–2, which is lower than the other two catalysts studied. Electrochemical impedance studies show that the composite material has the highest affinity toward OER of all of the materials investigated at several potentials. Chronoamperometric and chronopotentiometric investigations reveal short-term stability for the composite, where instability was observed for the comparison materials. This research is the first observation of transition-metal/graphene QD nanocomposites for electrocatalysis. These observations, along with high stability, serve as an exciting starting point for the foray into earth-abundant transition-metal QD-based electrocatalysts for clean energy and environmental applications.

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The development of efficient electrocatalysts for the oxygen evolution reaction (OER) is an enduring challenge toward the commercialization of electrochemical technologies such as water electrolysis and solar to fuel production. Although noble metal based electrode materials (e.g., Pt, IrO2, and RuO2 etc.) are active catalysts for the OER, their cost-effectiveness, scarcity and long-term stability have hindered the development of electrochemical commercial applications. In addition, there is a great interest in the design of cost-effective and efficient catalysts for the oxygen reduction reaction (ORR) for the creation of clean energy technologies. In this presentation, we will report on the synthesis of one-dimensional Co3O4 nanorods, two-dimensional nanosheets, and three-dimensional nanocubes. The formed 1D, 2D, and 3D Co3O4 were systematically probed using a structure sensitive electrochemical OER, revealing that the 2D nanosheets exhibited higher catalytic activities in contrast to the 1D and 3D Co3O4, due to its high electrochemically active surface area and rich oxygen deficiencies. In addition, nanostructured nickel cobalt metal oxides with different composition were synthesized and studied for oxygen reduction. The effect of the composition, morphology, and active sites of the formed cobalt based nanomaterials on their catalytic activity will be discussed.